Fan belt, drive sheave, and V-belt alignment field guide for HVAC

What a belt-drive fan is and why the setup is the job

A belt-drive fan moves air with a motor that never touches the fan wheel. The motor spins a sheave, a V-belt loops over to a second sheave on the fan shaft, and the belt carries the torque across the gap between the two shafts. That arrangement lets you set the fan to almost any speed by picking the two sheave sizes, which is why belt drives still run on most older air handlers, large exhaust fans, and cooling towers.

The setup is the whole job. Three things decide how the drive behaves: the belt tension, the sheave alignment, and the pitch diameters of the two sheaves that set the speed. Get those right and a drive runs for years on a little grease and a yearly look. Get them sloppy and the same drive wastes energy, throws belts on the first hot afternoon, and chews out the bearings on both shafts.

A loose, misaligned drive is the quiet capacity loss on an air handler. The belt slips, the fan turns slower than the design assumed, and the airflow at the registers drops while nobody touches the controls. The air-handling-unit guide covers where the fan section sits in the unit. This guide is the drive that turns it, and the fan-laws guide is what that speed does to the air.

The drive: motor sheave, fan sheave, belt, and bushing

A V-belt drive is four parts working together. The motor sheave, called the driver, is the pulley on the motor shaft. The fan sheave, called the driven, is the pulley on the fan shaft. The V-belt rides in the grooves of both and transmits the power. The fifth piece, easy to forget until it spins loose, is the bushing that locks each sheave to its shaft.

The belt does not ride on the bottom of the groove. A V-belt wedges into the angled sides of the sheave groove, and the wedging is what grips. That is why a worn groove or a glazed belt slips even at correct tension: the sidewall contact is gone and the belt is riding on the floor of the groove. The sheave wears too, not just the belt, and a sheave with a shiny, dished groove bottom needs replacement no matter how new the belt is.

Most sheaves mount on a tapered bushing rather than straight onto the shaft. A QD bushing or a taper-lock bushing slips over the shaft on a key, and bolts pull the sheave onto the taper to clamp it tight. The bushing is how you set the sheave's position on the shaft for alignment, and it is also where a sheave comes loose and wobbles when the bolts are not torqued. Know which bushing you have before you order a sheave, because the bushing series and bore have to match.

Driver sheave

The sheave on the motor shaft, sometimes the adjustable variable-pitch one

Driven sheave

The sheave on the fan shaft, usually a fixed sheave

Pitch diameter (PD)

The effective diameter where the belt rides in the groove, used for the speed ratio

QD / taper-lock bushing

A split tapered bushing that clamps a sheave to the shaft with bolts and a key

How do you set fan speed on a belt drive?

Fan speed comes from the ratio of the two sheave pitch diameters. The fan turns at the motor speed multiplied by the motor sheave pitch diameter divided by the fan sheave pitch diameter. A motor running 1750 RPM with a 4 in motor sheave and an 8 in fan sheave turns the fan at 1750 times 4 divided by 8, which is 875 RPM. The fan runs slower than the motor because the driven sheave is larger.

Pitch diameter, not outside diameter, is the number that counts. The belt rides down in the groove, so the effective diameter is a little smaller than the rim, and the manufacturer's pitch diameter is the figure you put in the ratio. Use the rim measurement and your speed calculation drifts off by a few percent, which on a fan is enough to miss design airflow.

Changing a sheave is how you change the fan's CFM on a belt drive. A smaller motor sheave or a larger fan sheave slows the fan and cuts the air; the reverse speeds it up. The fan laws set the price for that change, and the price is steep on the power side. Airflow tracks speed one for one, but brake horsepower climbs with the cube of speed, so a sheave change that bumps the fan 10 percent for more air can pull the motor toward overload. The fan-laws guide works that math; the rule on the drive is to clamp the motor amps after every sheave change.

Speed ratio

Motor sheave PD divided by fan sheave PD, the factor that sets fan RPM from motor RPM

Variable-pitch sheaves vs fixed sheaves

A variable-pitch sheave lets you dial the fan speed in without swapping hardware. It is an adjustable motor sheave with a threaded, movable flange. Open the flange and the belt rides deeper, which shrinks the pitch diameter and slows the fan. Close it and the belt rides higher, raising the pitch diameter and speeding the fan up. That adjustment range is how a balancer trims the air to design during test and balance.

The catch is that a variable-pitch sheave is a balancing tool, not a permanent setting. The movable flange is less balanced than a solid sheave, it wears faster, and it tends to drift. Common practice is to set the speed with the variable-pitch sheave during TAB, read the final pitch diameter, and then replace it with a fixed sheave of that diameter once the speed is proven. The fixed sheave runs smoother, lasts longer, and is easier on the belt.

Where a variable-pitch sheave stays in service, treat it as a wear item and check it. Lock rings back off, the flange creeps, and the fan speed wanders off the balanced point. If a unit was balanced two years ago and the air is now low, the adjustable sheave that crept is a prime suspect before you blame the duct.

Belt tension is the one that gets a drive killed

Belt tension is where most belt-drive failures start, in both directions. Too loose and the belt slips in the groove. Slipping belts glaze hard and shiny on the sidewalls, squeal on startup, run hot, and quietly lose fan speed and airflow, because a slipping belt is not delivering full motor speed to the fan. A slipping belt also burns itself up: the friction polishes the contact face until it cannot grip at all.

Too tight is the other failure, and it is the more expensive one. A belt cranked past spec puts a steady side load on the motor shaft and the fan shaft, and that load runs straight into the bearings. Overtensioned drives spin out fan bearings and motor bearings ahead of schedule, bend shafts over time, and stretch the belt's cords. The tech who reasons that tighter is safer is the one buying bearings every season.

Correct tension is the narrow band between those two, tight enough not to slip under the worst-case load, loose enough not to overload the bearings. There is no universal number for it, because it depends on the belt section, the sheave diameters, the span, and the load. The belt and sheave manufacturer publishes the tension or deflection force for the specific drive, and that figure governs. Set tension to the manufacturer's value, not by feel, and not by how the last belt felt.

How do you tension a fan belt?

Tension a V-belt by the force-deflection method, with a belt tension gauge, against the manufacturer's number. First measure the belt span, the straight free length between the two sheaves where the belt is not touching a groove. The target deflection is about 1/64 in for every inch of span. A 32 in span deflects roughly 1/2 in; a 16 in span deflects about 1/4 in.

Then push down at the center of the span with a belt tension tester and read the force it takes to deflect the belt that far. That force has to land inside the range the belt manufacturer lists for the belt section and the small sheave diameter. Below the range the belt is loose; above it the belt is over-tensioned. Snug the belt up or back it off and re-check until the deflection force is in band. A Gates Sonic Tension Meter or similar instrument reads belt span tension directly and skips the deflection geometry, which is faster on a multi-belt drive.

Adjust tension by moving the motor on its base, not by prying the belt over the sheave. Most fan motors sit on a slotted or jackscrew base for exactly this. Loosen the motor bolts, drive the motor away from the fan to take up slack, set the deflection force, then re-tighten the base bolts and re-check, because tightening the base can shift the tension. Never roll a new belt over the sheave rim to install it. That nicks the cords and the belt fails early. Slacken the drive, lay the belt in, then tension it.

Re-tension after the first day of run-in

New belts seat and stretch in their first hours of running, and a belt set correctly at install is loose by the next day if you do not come back to it. This is the single most skipped step on belt-drive service, and it is the reason a belt replaced last week is squealing this week.

As a new belt runs, it settles fully into the grooves and the tension cords pull into alignment under load, and the belt gives up most of its initial stretch in the first 24 to 48 hours. A common practice is to run the drive, then re-tension once after the first 20 minutes or so and again after the first day or two of operation. After that the belt holds tension and falls into the normal inspection schedule.

On a job where you cannot come back the next day, at least run the fan, shut it down, and re-tension before you leave. One run-in check beats none. Write the install date and the planned re-tension on the unit log or the equipment tag, because the re-tension is invisible work that nobody remembers to schedule. The belt that throws or burns up three weeks after a replacement almost always never got that second pull.

Why does sheave alignment matter?

Misaligned sheaves destroy belts and bearings faster than almost anything else on the drive. When the two sheaves are not in the same plane, the belt is forced to twist and rub its way through every revolution, which heats it, wears the sidewalls unevenly, and pulls it toward the edge of the groove. The same off-axis pull puts a thrust load on the bearings that they were never sized for.

Misalignment comes in three forms, and a real drive usually has some of each. Angular misalignment is when one shaft is tilted relative to the other, so the sheave faces are not parallel. Parallel, or offset, misalignment is when the shafts are parallel but the sheaves sit at different distances along them, so one groove is set in or out from the other. Twist is one sheave rotated about a vertical axis from the other. All three force the belt to track crooked.

You read misalignment on the belt before you read it with a tool. A belt that wears heavily on one sidewall, frays along one edge, or flips and rolls in the groove is telling you the sheaves are out. So is a belt that runs hot to the touch right after shutdown with correct tension. Industry guidance, including Gates' own, puts even small misalignment among the top causes of short belt life, so align the sheaves before you accept a new belt as a fix.

How do you align sheaves?

Align sheaves with a straightedge or a string for a quick field check, and with a laser sheave tool when you want it right. Lay a stiff straightedge, a long level, or a length of extruded aluminum across the faces of both sheaves. When the sheaves are aligned and the same width, the straightedge touches at four points: the top and bottom of the near sheave and the top and bottom of the far one. A gap at any of those points shows the misalignment. A gap top-to-bottom on one sheave is angular; a gap between the two sheaves is offset.

The straightedge works only when both sheave faces are true reference surfaces and the sheaves are the same width. When they are not, or when you need precision on a large drive, a laser sheave alignment tool is the better choice. It magnets onto one sheave's groove and projects a line onto a target in the matching groove of the other sheave, and you adjust until the line lands on the reference marks, which catches angular, offset, and twist at once.

Correct alignment by moving the sheaves on their shafts and shimming the motor, not by forcing the belt. Loosen the bushing and slide a sheave in or out to fix offset. Shim the motor feet or square the motor on its base to fix angular and twist. Check alignment after you set tension and again after you torque everything down, because tightening the bushing bolts and the motor base can both shift a sheave a little. Align it, tension it, then confirm both.

Belt types: classical, narrow, cogged, and banded

V-belts come in a few families, and the drive was designed around one of them. Classical V-belts run in the A, B, C, and D sections, sized by their top width, and they are the common older HVAC belt. Narrow belts, the 3V, 5V, and 8V sections, carry more power in a smaller belt and sheave and show up on newer and higher-horsepower drives. The cross-section stamped on the belt has to match the sheave groove, because a belt that does not seat correctly in the groove rides wrong and wears fast.

Cogged belts, marked with an X such as AX, BX, or CX, have notches molded into the underside. The notches let the belt bend more easily around small sheaves, run cooler, and flex with less loss, so a cogged belt typically runs a little more efficient than the plain belt of the same section. On a small-diameter fan sheave the cogged belt is the better pick and often the only one that lasts.

Banded belts join several V-belts side by side under one top band, which keeps them from flipping or jumping off on shock loads and pulsating drives. For multi-belt drives that throw belts, a banded set is one cure. Whatever the type, replace a belt with the same section and construction the drive was built for. The right belt for the job is the one the sheave grooves and the load were designed around, confirmed against the manufacturer's drive selection, not whatever is on the truck.

Do you have to replace all the belts in a set?

On a multi-belt drive, replace every belt at the same time with a matched set. A matched set is belts made to the same length within a tight tolerance, so they share the load evenly across the grooves. Mix a new belt with worn ones and the lengths no longer match. The new, shorter belt rides tight and carries far more than its share while the stretched old belts loaf, so the new belt overloads, burns out, and takes the rest down with it within weeks.

This is false economy that looks like a saving. The shop swaps the one belt that broke, the customer pays for one belt, and the tech is back inside a month replacing all of them anyway, plus the call. The cheaper path is to pull the whole set the first time and put on a matched set.

Buy belts marked as a matched set, or at least the same brand, section, and length code from the same source. Modern construction has tightened the spread between belts of the same part number, so some drives tolerate a mix better than they used to, but the rule on the job stays the same: a multi-belt drive gets all its belts replaced together, every time. One new belt among old ones is the call you make again soon.

Mounting the sheave: QD and taper-lock bushings

The sheave is only as true as the bushing under it. A QD or taper-lock bushing clamps the sheave to the shaft on a key, and getting it square and tight is what keeps the sheave from wobbling and the alignment from drifting. Set the key in the keyway first, slide the bushing and sheave onto the shaft, and start the bushing bolts loose so you can still position the sheave for alignment.

Pull the sheave on square, then torque the bolts. The bushing bolts have to come up to the manufacturer's torque, in sequence, a little at a time around the pattern, the same way you would a flange. Snug them unevenly and the sheave pulls on cocked, which throws the alignment and runs the belt crooked no matter how well you set the straightedge. A torque wrench belongs on these bolts, not a guess.

A loose bushing announces itself. The sheave wobbles, the belt flutters, and you often find a fretted, rusty smear of fine powder worked out of the bore and the key, the tell that the sheave has been moving on the shaft. By then the keyway may be damaged. Catch a loose bushing early by checking the bolt torque on the PM, because a sheave that has hammered its keyway oval is a shaft or sheave replacement, not a re-torque.

The bearings the belt loads

Belt tension does not disappear into the belt; it loads the bearings on both shafts. The fan shaft rides in pillow-block bearings, the motor has its own, and the side pull from the belt is a steady radial load on all of them. That is why overtensioning is a bearing killer and why correct tension is as much about bearing life as it is about belt life.

Grease the fan bearings on the schedule and to the amount the bearing maker calls for, not until grease pours out the seals. Over-greasing blows seals and runs the bearing hot, which is the opposite of the goal. A bearing starting to fail growls, runs hot, and shows up on a vibration check before it seizes, and a seized fan bearing with the motor still driving the belt is how you get a stalled fan and a cooked motor.

The bearing and lubrication detail belongs to the motor and AHU service, covered by topic in the air-handling-unit guide. The point on the drive is narrow: set tension to spec so the bearings carry the load they were sized for, and treat a hot or noisy bearing as the early warning it is.

The belt guard and lockout come first

Nothing on the drive gets touched until the motor is locked out. A belt drive is exposed rotating machinery with real pinch points where the belt enters the sheave, and a fan that starts on an automatic call or a remote command while your hand is on the belt takes fingers. De-energize the motor at its disconnect, lock it, tag it, and verify it is dead before any belt or sheave work. On a fan that can free-wheel in the duct draft, confirm the wheel is stopped too.

Lockout/tagout is the rule, not a suggestion. The fan may be controlled by a building automation system that has no idea you are inside the cabinet, and a thermostat call or a scheduled start does not care about your hands. The lock on the disconnect is the only thing that does. Do not rely on a control switch or a stopped fan; the energy-isolating device gets the lock.

The belt guard goes back on, every time. OSHA requires guarding on belts, sheaves, and rotating shafts that workers can reach, and the guard is also what keeps a thrown belt from whipping someone. A drive running without its guard is an open citation and an open injury. If the guard is bent so it rubs the belt, fix the guard. Do not run the drive bare because the guard is in the way.

Why does a fan belt squeal, slip, or throw?

A squealing belt is almost always loose or glazed. The chirp or squeal on startup is the belt slipping against the sheave as the fan's inertia loads it, and the fix is tension first, then inspect for glazing. A belt with a hard, shiny, slick face has glazed from slipping and will keep slipping even after you tension it, because the grip surface is polished off. Glazed belts get replaced, not just tightened, and you re-tension after run-in or you will be back.

Fast or one-sided wear points at the sheaves. A belt worn heavily on one sidewall, or that fries one edge, is running misaligned. A belt worn thin and bottoming in the groove, or a sheave with a bright dished groove, means the sheave is worn out and a new belt in a worn groove will fail quickly too. Check the grooves with a sheave gauge, and when the groove is worn, replace the sheave with the belt.

A thrown belt has a short list of causes: a loose belt, bad alignment, a worn groove, a foreign object, or a pulsating shock load the single belts cannot hold. Find which one before you just throw another belt on, or it lands on the floor again. Vibration is its own signal. A drive that shakes can be a loose bushing, an unbalanced or damaged sheave, mismatched belts in a set, or a failing bearing, and the cure depends on which, so read the rest of the symptoms before you reach for a part.

Reading the wear on a belt

A worn belt is a record of what the drive did to it, and a pulled belt tells you what to fix before the next one goes on. Glazing, a hard glassy sheen on the sidewalls, is slip from low tension. Cracking across the underside, the cogs, or the back is heat and age, often from a too-small sheave, high temperature, or a belt that has simply run out its life. Both mean the belt is done.

Edge and sidewall wear point at geometry. Wear concentrated on one sidewall is misalignment pulling the belt to one side. A belt frayed or shredded along an edge has been rubbing a guard, a bracket, or a misaligned groove. A belt worn down so it rides on the bottom of the groove, with the top sitting below the sheave rim, has stretched out or the groove is worn, and either way the wedge grip is gone.

Spin-burn or a flat spot melted into the face comes from a belt that slipped while the sheave kept turning, usually a seized fan, a jammed wheel, or a belt that lost all tension. That belt failed in seconds, not over time, and the cause is mechanical, not the belt. Pull the belt at each PM, flex it, and look at all four surfaces before you decide what the drive needs. The belt will tell you whether the problem was tension, alignment, the sheave, or just age.

Where a belt drive loses energy

A V-belt drive is efficient when it is set up right and wasteful when it is not. A well-tensioned, well-aligned standard V-belt runs in the mid-90s percent efficient when new. Let it slip, run it loose or misaligned, or let it glaze, and that efficiency falls off, because the energy lost to slip and flex goes out as heat in the belt instead of into the fan.

Cogged belts cut some of that loss. The notched underside flexes with less internal friction than a solid belt and runs cooler, so a cogged belt of the same section typically picks up a point or two of efficiency over its plain equivalent, and the savings run for the life of the belt. Synchronous, or timing, belts go further by using teeth that mesh with a grooved pulley so the drive cannot slip at all, and they run efficient near 98 percent, though they are noisier, need precise alignment, and suit fixed-speed drives where the noise is acceptable.

The bigger savings on most drives is not the belt type, it is the maintenance. A slipping belt is paying full power for less than full airflow, and a misaligned drive is burning energy as heat at the sheaves. Set the tension, align the sheaves, and a standard drive captures most of what is there to capture. Where a drive runs constantly and the load justifies it, a cogged or synchronous upgrade is the next step, but a loose belt on a synchronous pulley still wastes air.

Belt drive vs direct drive and EC plenum fans

New equipment is moving away from belts. A direct-drive fan couples the wheel straight to the motor shaft, so there is no belt, no sheaves, and no alignment to maintain. Pair that with an EC, electronically commutated, motor and the fan's speed is set electronically, which gives the speed control a belt drive needed a sheave change to get. The whole class of belt problems in this guide, slip, tension, alignment, thrown belts, matched sets, goes away.

EC plenum fans and fan arrays, often called fan walls, are the common form on newer air handlers. An array of small direct-drive EC fans replaces one large belt-drive fan, adds redundancy, and trims speed at part load without a sheave or a drive. The air-handling-unit guide covers the fan-wall arrangement and the fan-laws guide covers why slowing those fans saves so much. The trade-off is that an EC motor or drive is an electronic repair, not a belt and a wrench, and it is a board swap rather than a five-dollar part.

Belt drives are not gone. They still run on a huge installed base of air handlers, exhaust fans, and cooling towers, and they are cheap, field-serviceable, and easy to re-speed with a sheave. Knowing the drive keeps those running, and knowing where the industry is headed tells you when a failed drive is better retrofitted to direct drive than rebuilt.

Commissioning and the belt-drive PM

A belt drive gets set right once at startup and then checked on a schedule. At commissioning, align the sheaves, install the belt without prying it over the rim, tension it to the manufacturer's deflection force, run it, and re-tension after the run-in period. Clamp the motor amps at the final speed and confirm they sit under the nameplate with the service factor as margin, not as the plan. Record the sheave sizes, the belt, the tension, and the amps as the baseline.

The recurring PM is short and it pays. Lock out the motor. Pull the guard. Check belt tension and reset it to spec. Inspect the belt on all four faces for glazing, cracking, and edge wear, and replace a worn belt or set. Check sheave alignment with a straightedge. Confirm the bushing bolts are at torque and the sheave is not wobbling. Listen and feel the bearings for heat and noise, grease them on schedule, and put the guard back on before you restore power.

On a unit that has lost airflow, the drive is the first place to look before the duct or the controls. A slipping belt, a crept variable-pitch sheave, or a fan running slow on a stretched belt all read as a system that lost air, and all of it is a fifteen-minute check at the drive. Catch it there and you save chasing it through the whole air side.

Belt drives on AHUs, CRAH units, and cooling towers

The same drive shows up across HVAC, and the stakes change with the application. On a comfort air handler a thrown belt is a hot building and a service call. On a data-center CRAH or a critical-process unit, an older belt-drive air handler that loses its belt loses cooling to a room that cannot tolerate it, which is why the data-center world pushed hard toward direct-drive EC fan arrays with redundancy.

Where belt drives still serve critical or hard-to-reach equipment, the maintenance discipline has to be tighter, not looser. Cooling-tower fans run belt drives in a wet, corrosive, hard-access spot where alignment and corrosion-resistant components matter and where a failure means a tower section down. Large exhaust and makeup-air fans are often belt-driven for the speed flexibility. The drive does not care what it serves, but the consequence of a sloppy drive scales with what is downstream.

The rule across all of it is the same: matched belts, correct tension, true alignment, torqued bushings, and a guard, checked on a schedule that matches how much the equipment costs you when it stops. The more critical the load, the shorter the inspection interval, and the stronger the case for retrofitting a failing belt drive to direct drive.

What to document

A belt drive set up without a record is a drive the next tech has to reverse-engineer. Write down the sheave sizes and the belt so the next replacement is the right part, and the tension and amps so anyone can tell whether the drive drifted. The table below is the field version of the drive's record.

Common mistakes

• Running the belt too loose, so it slips, glazes, squeals, and quietly loses fan speed and airflow.
• Cranking the belt too tight, which overloads the motor and fan bearings and bends shafts over time.
• Leaving the sheaves misaligned, so the belt and the bearings wear out fast.
• Skipping the re-tension after the first 24 to 48 hours of run-in.
• Replacing one belt of a multi-belt drive instead of the whole matched set.
• Fitting the wrong belt section or construction for the sheave grooves and the load.
• Rolling a new belt over the sheave rim to install it, nicking the cords.
• Running the drive with no belt guard, or working on it without locking out the motor.
• Ignoring the wear signatures on the old belt and just bolting a new one on the same bad setup.
• Leaving a worn or wobbling sheave or a loose bushing under a brand-new belt.

Field checklist

Standards and references

The controlling document for any specific drive is the belt and sheave manufacturer's data. Gates, Browning, and similar makers publish the belt section, the sheave pitch diameters, and the tension or deflection force for the drive, along with the deflection-distance method described here. Set tension and pick belts to that data, because the right force depends on the belt section, the small sheave diameter, the span, and the load, and no rule of thumb replaces the manufacturer's number.

Fan air performance and the fan curve trace to AMCA, the Air Movement and Control Association, which sets how a fan's airflow, pressure, and power are rated, so a sheave or speed change ties back to the AMCA-rated curve for the fan. Bearing life and vibration acceptance follow the bearing maker's data and the ISO vibration standards for rotating machinery, which is the basis for judging whether a drive's vibration is normal or a warning.

Guarding of belts, sheaves, and rotating shafts is governed by OSHA, and the energy-isolation requirement to lock out the motor before drive work comes from the OSHA lockout/tagout rule. Standards are adopted and amended by jurisdiction, and equipment data varies by model, so confirm the manufacturer's current figures and the adopted regulations before you treat any number here as fixed. The figures in this guide are typical practice, not a substitute for the drive data or the project specification.

Units, terms, and conversions

A belt drive crosses a few unit systems and carries a few names for the same part, so the same thing can read differently across a nameplate, a parts catalog, and a balancing report.

Sheaves and pulleys are the same component; pitch diameter (PD) is in inches in the US and millimeters in metric catalogs. Speed is RPM at the shaft. Belt tension is read as a deflection force in pounds-force (lbf) or newtons (N), and belt span and deflection are in inches or millimeters. Bushing bolt torque is in pound-feet (lb-ft) or newton-meters (N-m). Belt sections carry letter codes, A, B, C, D for classical and 3V, 5V, 8V for narrow, with an X suffix such as AX or BX for cogged construction.

Sheave / pulley

The grooved wheel a V-belt rides in; driver on the motor, driven on the fan

Pitch diameter (PD)

The effective diameter where the belt rides, used in the speed ratio

Variable-pitch sheave

An adjustable sheave whose movable flange changes pitch diameter to trim fan speed

Deflection force

The force to deflect the belt span 1/64 in per inch of span, checked against the maker's range

Matched set

Belts within a tight length tolerance, replaced together on a multi-belt drive

QD / taper-lock bushing

A split tapered bushing that clamps a sheave to the shaft with bolts and a key

Cogged belt

A notched V-belt (AX, BX, CX) that flexes easier, runs cooler, and gains a little efficiency

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