Belts, Couplings and Drives: How Power Gets Lost Between the Motor and the Fan

• V-belts reach 95 to 98 percent efficiency when new but can lose up to 5 percent by slipping. Cogged belts gain about 2 percent, and synchronous belts hold about 98 percent.
• Slip shows itself: squeal at start up, glazed sidewalls, black belt dust under the guard and a sheave running warmer than the motor.
• Synchronous belts do not slip, so the fan speeds up. Sprockets must be selected for the new speed, or energy use can rise instead of fall.
• Couplings report misalignment: rubber crumbs under a jaw coupling, grease leaking from a grid coupling and cracked discs in a disc pack.

The drive transmits the motor's power to the fan or pump. On belt drives it also sets the speed through the ratio of the pulleys, called sheaves. Everything the drive loses as slip, heat or friction is power the motor pays for and the air or water never gets.

• Belt drives: V-belts, cogged belts or synchronous belts on sheaves. Common on air handling unit (AHU) fans and many older computer room air handlers (CRAH units).
• Couplings: a flexible joint between motor and pump shafts that absorbs small misalignment.
• Direct drive: the fan sits on the motor shaft, with no belt or coupling at all.

Drives are built to be the wear part. A belt or coupling element costs far less than a motor or pump, and it is meant to give way first. The aim is to replace it on your schedule, before it fails on its own.

Drives also tell you about the machine. A coupling that sheds rubber is often reporting misalignment, and a belt that keeps slipping may be reporting a seized bearing or an overloaded fan.

V-belt drives can reach 95 to 98 percent efficiency at installation, according to DOE Motor Systems Tip Sheet #5. Efficiency can drop by as much as 5 percent over time if the belt slips because it is not retensioned (DOE).

• Standard V-belts: 95 to 98 percent when new, falling as they stretch and slip.
• Cogged, or notched, belts: run cooler, last longer and are about 2 percent more efficient than standard V-belts.
• Synchronous, or toothed, belts: about 98 percent efficient, and they hold it across a wide load range because they do not slip.

What the difference is worth

The DOE works an example. A continuously running 100 horsepower (hp) supply fan motor uses 527,000 kilowatt hours (kWh) a year. Replacing a 95 percent efficient V-belt with a 98 percent synchronous belt saves about 16,130 kWh a year.

The US General Services Administration (GSA) tested synchronous and cogged belts on real building fans in a field study (GSA). Fans that run around the clock are where drive efficiency adds up fastest, and the savings show in a building's ENERGY STAR score.

A slipping belt announces itself through sound, heat and wear long before it breaks. The signs are visible and audible on a normal walk past.

• Squeal: at start up or when the load changes, as the belt slides over the sheave.
• Glazed sidewalls: shiny, hardened belt sides polished by slipping.
• Black belt dust: rubber worn off the belt and collected under the guard.
• Heat: a belt and sheave noticeably warmer than normal, or warmer than the motor.
• Dished grooves: sheave grooves worn into a dish, so the belt rides low and slips more.

Heat is easy to compare with thermal imaging: the sheave and belt against the motor frame beside them, or against the same drive on a sibling unit. Acoustic listening catches the squeal and any change in it from one round to the next.

Slip is a symptom as well as a fault. A belt that slips after being tensioned correctly may be reporting a bearing that has started to drag, covered in Part 1.

A fan belt should be at the lowest tension at which it does not slip at peak load. Tighter than that only adds load to the motor and fan bearings; looser lets it slip, heat and wear.

Most belt stretch happens early. That is why tension is checked after the break in period of a new belt and then periodically after that (DOE).

Checking tension properly needs a tension instrument and the unit locked out. A walk past cannot measure tension, but it can catch the signs that tension has been lost: squeal, glazing, dust and heat.

The check interval belongs in the preventive maintenance schedule, and each check belongs in the maintenance management system with the date and the method used.

Sheave misalignment wears one side of the belt, can roll a belt over in its groove, and wears multi belt sets unevenly. The belt is forced to enter and leave the grooves at an angle on every pass.

• One sided wear: the belt sidewall facing the misalignment wears faster than the other.
• Rolled belts: a belt can turn over in its groove and run on its side.
• Uneven sets: on a multi belt drive, some belts carry more load than others and wear out first.

Sheave alignment is checked with the unit locked out, using a straightedge across both sheave faces or a laser sheave alignment tool. The laser is faster and removes guesswork on long spans.

Correct the alignment before fitting new belts. Otherwise the new set starts wearing on one side from the first hour, and the money spent on belts buys only a few months.

Sheave misalignment and shaft misalignment are related problems. Shaft misalignment across a coupling is covered in Part 2, including how it shows in vibration.

All the belts in a matched set should be replaced together because a new belt is shorter than stretched old ones. Mixed into a used set, the new belt carries most of the load.

That overloaded new belt wears out quickly, and the old belts run slack and slip. The drive ends up with the worst of both.

Replace the whole set at once, and check the sheave grooves at the same time. New belts on dished grooves ride low and slip from the first day.

Record the belt part number and the date on the drive or in the work order. The next technician can then order a matched set and knows how long the last one lasted.

Belt vibration shows at the belt frequency, which always sits below running speed, often with harmonics. That separates it from imbalance, which sits at exactly 1× running speed.

Belt frequency is the belt's speed divided by its length: how many times per second the belt completes a loop. As a formula, it is pi times the sheave pitch diameter times the rpm, divided by the belt length times 60.

In vibration analysis, a peak below running speed with harmonics points to the belt. A peak at running speed points to the sheave or rotor. A wobbling sheave and an unbalanced rotor both show at running speed, so they need phase readings to separate, as covered in Part 2.

Each coupling type fails in its own way, and most of them leave visible evidence before they fail. The four common types in mechanical rooms:

• Elastomeric jaw, or spider, couplings: shed rubber crumbs when misaligned or worn out. A small pile of crumbs under the guard is the first sign.
• Grid couplings: need grease. A leaking cover seal means the grid is running dry.
• Disc pack couplings: thin metal discs that can crack under excess misalignment.
• Gear couplings: need lubrication, and wear quickly without it.

Couplings report the alignment of the machine. Coupling heat and misalignment are often the same problem seen two ways. A warm or crumbling coupling is a reason to check alignment, not just to replace the element.

Guarding

Belts, pulleys, shafts and couplings must be guarded under OSHA 29 CFR 1910.219, the federal rule for mechanical power transmission. A missing or damaged guard is a finding in its own right; our guide to OSHA rules in commercial buildings covers the wider picture.

When a motor's own cooling fan is damaged, the motor either vibrates or overheats. A totally enclosed fan cooled (TEFC) motor relies on a shaft mounted fan under a shroud. That fan blows air over the finned frame.

• A cracked fan blade: adds imbalance at 1× running speed, which looks like a rotor problem in a vibration reading.
• A blocked shroud: starves the frame of cooling air, so the motor runs hot even at normal load.

Both are visible from the outside. A shroud packed with debris or a fan guard that rattles is worth a note on every round. Motor overheating is covered in depth in Part 6.

Not always. Synchronous belts and direct drive fans both remove slip, but each brings its own trade offs, and the DOE spells out the cautions for synchronous belts (DOE).

• Speed rises: because synchronous belts do not slip, the driven fan or pump speeds up. Sprockets must be selected for the new speed, or energy use can go up instead of down.
• Noise: synchronous belts can be noisier than V-belts.
• Shock loads: sudden torque can shear their teeth. Cogged belts are the better choice where shock loads or vibration damping matter.

The speed point is the one most often missed. After a switch to synchronous belts, recheck fan speed and airflow. Otherwise the gain can be eaten by a fan that now moves more air than needed.

The move to direct drive

Newer AHUs and computer room air handlers increasingly use direct drive electronically commutated (EC) fans and fan arrays. That removes belts entirely, but it adds more motors to watch, each with its own bearings.

Drives are visible, so they get blamed first. Three lookalikes cause most of the confusion.

• Belt squeal versus a dry bearing: belt squeal comes and goes with start up and load changes. A dry bearing reads steadily high on airborne ultrasound. Covered in Part 1.
• A wobbling sheave versus imbalance: both show at running speed. Covered in Part 2.
• Coupling heat versus misalignment: often the same problem seen two ways. Covered in Part 2.

Pump noise that changes with flow is a different problem again, usually cavitation, covered in Part 3. This is why no single sensor is enough: sound, heat and vibration each rule out different suspects.

A drive failure costs energy while it develops and airflow or flow when it fails. The energy cost is quiet and continuous; the failure cost is sudden.

• Energy: wasted drive efficiency on fans that run around the clock, every hour of every day.
• Office building: lost airflow and comfort complaints when a belt fails, and a thrown belt takes the AHU offline. Routine checks are covered in our guide to AHU and plant maintenance in office buildings.
• Data center: CRAH units with belt drives lose cooling capacity at the point of failure, and a failed coupling stops a pump outright. Our guide to cooling loads nobody owns covers who should own that risk.

Repeat belt or coupling failures on the same machine point to a cause upstream, such as misalignment or a dragging bearing. Our guide to recurring HVAC failures covers how to find it before it becomes deferred maintenance.

A walk past can see and hear most drive problems, which makes drives one of the most inspectable parts of a fan or pump. It cannot measure belt tension or groove wear. Each finding carries a label for how much it shows.

• Directly observable: belt dust, glazing, a turned over or missing belt, rubber crumbs, grease leaks, and a missing or damaged guard.
• Strong indicator: a sheave or belt running warmer than the motor end, and squeal on start up.
• Suspected: actual belt tension and sheave groove wear. Both need a tension gauge or groove gauge with the unit locked out.

That is the limit of any route based check: it decides which drive earns a lockout and a gauge. A camera records belt and guard condition through the guard openings. A thermal reading with a reference in the same frame shows a hot sheave beside the cooler motor.

On drives with variable speed, the drive panel readouts give the speed and current to record with each finding. Our predictive maintenance guide covers turning those readings into planned work.

• Black dust under the belt guard.
• Shiny or cracked belt sidewalls visible through the guard.
• Squeal at start up.
• Belt sets: all belts the same size and wear, and none turned over.
• Temperature: sheave and belt compared with the motor.
• Couplings: rubber crumbs under elastomer couplings, and grease leaking from grid coupling covers.
• Guards in place and complete.
• Motor fan shroud intact and clear.

Coming Soon

1. 1. Which of your fans still run on standard V-belts?
2. 2. When was belt tension last checked on each AHU, and by what method?
3. 3. When a belt was replaced, was the whole set replaced?
4. 4. Were fan speeds rechecked after any switch to synchronous belts?
5. 5. Which coupling types are in your plant, and which need grease?

How can you tell if a fan belt is slipping?

Listen for squeal at start up or when the load changes, and look for glazed, shiny sidewalls and black belt dust under the guard. A belt and sheave running warmer than the motor is another sign.

How tight should a fan belt be?

A fan belt should be at the lowest tension at which it does not slip at peak load. Most stretch happens early, so tension is checked after the break in period and then periodically with a tension gauge.

How efficient are V-belts compared with synchronous belts?

The US Department of Energy puts new V-belt drives at 95 to 98 percent efficient, falling by up to 5 percent if they slip. Synchronous belts run at about 98 percent and hold it across a wide load range because they do not slip.

What causes black dust under a belt guard?

Black dust is rubber worn off the belt, usually by slipping or by sheave misalignment. It is a sign the belt needs tensioning, the sheaves need aligning, or both.

Why replace all the belts in a set at once?

A new belt is shorter than stretched old ones, so in a mixed set it carries most of the load and wears out quickly. Replacing the whole set keeps the load shared evenly.

What does rubber dust under a coupling mean?

Rubber crumbs under an elastomeric jaw coupling mean its spider is wearing, usually from misalignment or old age. Replace the element and check the shaft alignment, or the new one will wear the same way.

Do couplings need lubrication?

Some do. Grid and gear couplings need grease or lubrication, and a leaking grid coupling cover seal means it may be running dry. Elastomeric jaw couplings and disc pack couplings do not need lubrication.

Does OSHA require belt guards?

Yes. OSHA 29 CFR 1910.219 covers guarding of belts, pulleys, shafts and couplings on mechanical power transmission equipment. A missing or damaged guard should be fixed before anything else.

Why did fan speed change after switching to a synchronous belt?

Synchronous belts do not slip, so the fan turns closer to its full designed speed. Unless the sprockets were selected for the new speed, the fan moves more air and energy use can rise instead of fall.

• Imbalance, Misalignment, Looseness and Soft Foot: Four Faults, Four Vibration Signatures
• ENERGY STAR Portfolio Manager and Building Performance Standards: How Equipment Condition Drives Your Score
• OSHA Regulations for Electrical Rooms and Exit Routes in Commercial Buildings
• Thermal Imaging in Data Centers: Seeing Heat Before It Becomes Downtime

• US Department of Energy: Motor Systems Tip Sheet #5, V-belt, cogged and synchronous belts
• US General Services Administration: field study of synchronous and cogged fan belts