Relieving Tension

The V-belt is one of industry’s most efficient and popular means of power transmission. Properly selected, installed and maintained, quality V-belts will provide years of reliable service and prolong the life of any motor.

The single most important factor in providing optimum operating efficiency is maintaining proper belt tension. Most problems encountered with V-belts can be directly attributed to incorrect tension.

It’s also important to perform the necessary steps to properly maintain V-belt drives: cleaning the belts and sheaves, recognizing problems, and correcting tension on existing belts. As well, it’s important to follow the proper steps for installing new belts, including learning how to check belt and sheave wear, choose correct replacement belts, remove and apply belts and set the proper tension.

First, whenever inspecting any type of electrically driven machinery, be sure to lock out and tag the main disconnect of the equipment being worked on. Remove the belt guard and clean it thoroughly. This will prevent dirt and other foreign material from dropping from the underside of the guard onto the belts or the sheave grooves below.

Look for these telltale signs of premature belt failure. If the belt sidewalls are glazed and cracked, or have pieces missing, the cause may be too much heat generated by belt slippage. This condition is caused by extreme loads, low belt tension, drive stalling, or a poorly designed drive. Also inspect the top of the old belts for wear or gouges in the cover. This indicates that the belt guard may have been in contact with the belts and you need to adjust the guard placement.

V-belts should always be kept clean. A stiff bristle brush may be used to eliminate any dirt that has accumulated. Particles of dirt can embed themselves into the belts, making them coarse and rough and causing undue wear on the sheave. Oil and grease can be removed with a simple soap and water solution.

Oil and grease are the worst enemies of V-belts. They cause the belt material to soften and swell up, making them feel spongy to the touch. Once the physical properties of the belt begin to break down, belt failure is not far behind.

Under no circumstances should you ever use belt dressings. Dressings often have a solvent affect on rubber compounds, causing premature belt deterioration.

Next, visually inspect the sheave grooves for any foreign materials that may be lodged there. These particles could force the belts from their path in the grooves and cause them to turn over, or can cause groove wear that decreases the life of the sheaves. Clean with a safe solvent and rag. Look for any bent flanges or broken sheaves. To prevent rapid belt wear and early drive failure, replace these parts immediately.

Now check the tension of the old belts. Restart the equipment. Observe the running drive. It should operate with the tension side tight and the slack side with a slight bow, both at start-up and when operating at peak load.

Listen for a squeal when the drive is started or at peak load. A squeal means that the belts are too loose. If one or more of the belts is running loose, the drive needs to be retensioned or the belts should be replaced with a matched set.

While the engine is warm, turn the machine off, lock out and tag, and touch the sheave grooves. If they are hot, meaning that you cannot place your hand in the groove for any length of time, the belts are slipping and the tension must be increased.

To tighten the belt, loosen the motor hold-down bolts and increase the distance between the two shafts. On most drives, this is done by moving the motor mount back until both spans of the belts are snugged. Be sure to retighten the motor hold-down bolts.

Any time you make a change in the distance between the two shafts, it is imperative that alignment be rechecked. The easiest and most common method of determining sheave alignment is with a straight-edge. Simply place the straight-edge between the outside edges of the two sheaves. The straight-edge should make contact at two distinct points along the outside perimeter of both sheaves. Rotate the sheaves a half turn and check again. As before, the straight-edge should be making contact at two distinct points on both sheaves. Now repeat the entire process on the inside face of the sheaves.

On horizontal drives, use a level to check for angular misalignment. It is essential that both sheaves are in the same vertical plane and the belts are not twisted.

If, during your visual inspection, you have determined that new V-belts are needed, remove the old belts. Begin by loosening the motor hold-down bolts and sliding the motor base so you can easily remove the old belts. Never attempt to force or pry belts on or off a drive. This will cause the outer fabric of the belts to crack or it may rupture the inner cords. Either will result in a shortened belt life, turnover, or belt breakage.

Now that you have access to the sheaves, clean the sheave grooves thoroughly.

Use a sheave gauge to check for sheave groove wear. Select the proper gauge according to the sheave diameter and insert it into the groove. The gauge rim should rest on top of the outside diameter of the sheave and the inside should lie flush against the side wall of the groove. On multiple belt drives, be sure to check all the sheave grooves for uniformity.

A common pattern for groove wear is one where the side walls become concave or dished out. This occurs most often in the small sheave. A dishing out that measures more than 1/32 of an inch will cause the belts to lose their wedging action. The sheave should be replaced. Another indication of excessive sheave wear is a shiny sheave groove bottom. If the sheaves are in good condition, proceed with installation of the new belts.

It is critical that you install the correct belts. Be sure that all belt replacements are a matched set. Also, never replace just one belt in a set. New and used belts should never be used together. It is also recommended that you never mix belts from different manufacturers. Staying with one manufacturer will make identification and selection of belts much easier and result in better drive performance.

Take the belts and place them one at a time over the sheave grooves. Be sure the motor mount is sufficiently loosened so no undue force is needed to place the belts over the sheaves.

Now you’re ready to reset the tension. There are various tools to help you tension belts properly. An easy and accurate method is with a Tension-Finder tool. With the motor off, locked out and tagged, begin by applying enough tension to take the slack out of the belt. Next, use the Tension-Finder as a square and scribe a line across the belt. Place the first slot, or start slot, over the line. With the line in the start slot, attach the clip to the belt to serve as a stop. Simply tension the belt until the line moves from the start slot to the designated slot.

Remember, it should also be standard procedure to check the sheave alignment after any adjustment in the distance between the shafts. Run the drive for 15 to 20 minutes. This will give the belts time to seat themselves in the sheave grooves. Then recheck the tension. Over-tensioned belts can be just as much a problem as under-tensioned belts. Belts that run too tight put too much strain on the drive, causing the bearings to overheat.

Each belt should ride in the sheave groove so the top surface of each belt is just above the highest point of the sheave. This may vary slightly depending on the type of sheave and belt, but it is an important point to check before reinstalling the belt guard.

When you have correctly installed the new belts, properly set the tension and re-aligned the sheaves, you’re ready to reinstall the belt guard.

In review, as a part of your regular maintenance, clean your belts and sheaves and check belts for wear and correct tension. When installing new belts, be sure they are a matched set from the same manufacturer, and nev
er mix old belts with new.

Most importantly, take the time to accurately set the tension of your new belts, and always recheck sheave alignment any time you’ve changed the distance between sheaves.

When properly installed and maintained, your V-belts will deliver the superior performance for which they were designed.

Brent Elliott is a senior application engineer with Carlisle Power Transmission.