Proper Installation and Maintenance Can Prolong the Life of V-Belts
By John C. Robertson, maintenance reliability specialist

Step 1
Follow your company's safety work practices during the installation of the V-belts, including personal protective equipment policies and lockout and tag-out policies.

Step 2
Remove the safety guard from the V-belt drive area.

Step 3
Adjust the moveable plate toward the fixed component by using the adjusting screws to reduce the center-to-center distance of the driver-to driven sheaves. This reduces the tension on the belt and allows slack in the belt between the sheaves.

Step 4
Remove the old belts from the sheaves. Examine the operational surfaces to determine if any damage had forced the belts into an early demise. Look specifically for fabric wear on the sidewalls, reinforcing nylon cords, cracking caused by dry out, and oily surfaces.

Note: If any of the above symptoms are apparent, do not install any new V-belts until the root cause of the problem has been identified and corrected.

Step 5
Clean the sheaves of all foreign matter with a stiff brush that has bristles softer than the sheave surface material. Heavy-duty wire brushes can scratch the surface of the groove walls. These scratches can, in turn, tear up the V-belt's outer skin and systematically destroy the belt.

Step 6
Using the "go-no-go" slip gauges that can be obtained from a belt manufacturer, determine the condition of the V-groove in the sheave. This will accurately determine if the walls of the V-groove have been subjected to excessive forces caused by improper tension causing slippage and poor alignment between the driver and the driven shafting.

Step 7
If the sheaves do not meet these criteria or are damaged in other ways (chipped or cracked sidewalls), discard these defective parts and install new ones.

Step 8
Verify that the replacement belts are the correct size. Check with the "go-no-go" gauge to ensure the cross-section of the V-belt is compatible with the V section in the groove. The belt must ride in the groove with its top flat surface level with the outer periphery of the sheave.

Note: Never mix new and old belts regardless of the "new" look of the old one. Belts should always be installed in matched sets. This ensures that all of the replacement belts are exactly alike in all respects. Never mix belts from different manufacturers because they have different stretch characteristics, coefficients of friction, and cross-sectional areas. If the V-belts are not the same length, they will not carry the same amount of load. This will cause some of the belts to become overloaded and wear rapidly, shortening the life of the belt drive.

Step 9
Before installing the new belts, the following checks must be made:

• Check the TIR (Total Indicated Run-out) of both the driver and driven shafts. These should be within +/- 0.003". If the run-out reading exceeds this value, the shaft(s) must be straightened. This check must also be carried out on the outer rim of each sheave, as it is quite common to find the shaft hole in the hub drilled off-center causing damaging eccentricity. This eccentricity causes the belts to slacken off at the 3 o'clock position and to snap into tension at the 9 o'clock position during shaft rotation. This continual snapping action creates rapid belt and bearing deterioration.
  
• Check all of the hold-down bolts around the bedplate to determine if any soft-foot conditions exist. This reading should not be greater than 0.002".
  
• Check sheave alignment by placing a straightedge or a tightly drawn cord across the sheave faces so that it touches all four points of contact.
  
• Note: This method of alignment is only effective when the sheaves are a matched pair. If the sheaves are mismatched, there may be differences in the sidewalls' thickness, which will aggravate the misalignment. When this is the case, align the Vs with each other, as this is the perfect way to line up the belts. Misalignment causes uneven wear on one side of the belt, which causes it to roll over in the sheave, or it can throw the entire load on one side of the belt, stretching or breaking the cords. Therefore:
  
• Parallel the position of the sheave shafts.
  
• Correctly align the grooves in the sheave.

Step 10
Install the new belts on the sheaves so that the slack sides of all belts are on the same side, either top or bottom, of the drive.

Caution: Under no circumstances install the belts by prying them onto the sheaves with a screwdriver or any other forcible method. This will damage the internal cords of the belts and possibly break off the rim of the sheave's sidewalls, which would cause unbalance of the rotating components. The motor must always be detensioned enough to allow the belts to be removed or installed without forcing them.

Step 11
Adjust the tensioning screws to pull the motor away from the driven unit until the belts are correctly in tension. The following formula is used for determining the correct tension of the belts:

Tension load = The distance in inches between the axes of the driver and driven shafts x 1/64"

For example, if the distance between the centers of the driver shaft and the driven shaft is 64 inches, the belt deflection load will be:

Deflection Load = 64 inches x 1/64 inch  = 1 inch of deflection

Step 12
When the belts are correctly in tension, paint a thin, narrow line across the belts' top surfaces at 90 degrees to the length. (After the unit is started, a strobe light flashing on the belts at the operating frequency of the belts will show the painted line appearing as if it was stopped. Should there be any slippage, the belts that are slipping will be moving away from the line at various speeds according to their degree of looseness. This can be expected during the initial run-in period, but the belts must again be retensioned to allow the correct deflection. This may have to be repeated until all of the slack is taken out of the belts.)

Step 13
Replace the safety guard before removing all lockout and tag-outs. Note: The safety guard should be constructed from extruded open mesh steel as this permits free passage of air to circulate across the belt area in order to keep the belts cool and allow heat to escape.    

Step 14
Start the unit and allow the belts to seat themselves in the grooves of the sheaves.    

Step 15
Stop the unit after a few hours to check the tension of all of the belts. (Refer to Step 12). Note: Before checking the belt tension, ensure all of the lockout procedure is in place.

Step 16
Restart the unit. Note: This is probably the most ignored task in belt installation, but it is a very important step in the operation and maintenance of V-belts. As such, it is worth repeating the following:

After the machine has run for 48 hours, the tension on the new belts should be checked and retightened to the correct midspan deflection setpoint. This process must be repeated until all of the stretch has been eliminated. Belts that squeal during acceleration or when operating at full load usually have slippage. Never add a lubricant to the belts. Squealing merely indicates that the belts need to be tightened. This will extend the lifespan of the belts and bearings immensely.

Avoid leaving old V-belts and other maintenance debris lying around after maintenance activities are completed. Collect waste products in an approved container and dispose of this waste according to established procedures.

This article is provided courtesy of Strategic Work Systems, Inc.

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