MRO Today - V.I.P.: V-belt Installation Procedure

V.I.P.
V-belt Installation Procedure helps you do the job right every time

by John C. Robertson

V-belts run longer and perform better if given proper care and attention during installation, and in particular, during the subsequent 48-hour running-in period.

The run-in period is a critical time for V-belts, especially if they are to last for a few years. During this time span, the initial stretch is removed from the belt. Also, the soft rubber surface of the belt’s outer envelope abrades away, and the belt settles deeper in the groove of the sheave. This causes the belt to run slack.

At this point, take up the slack on the new belts to avoid considerable slippage, frictional burning and other irreparable damage.

It’s important to check the belts often during the first days of operation and adjust to the correct tension until no signs of stretching are present. This eliminates early damage and promotes longer belt life.

This article provides a standard procedure for correctly installing a V-belt and the sheaves in which they operate. This, in turn, improves mechanical efficiency of the motor and driven mechanical equipment by reducing wear on rotating mechanical components.

This procedure provides general guidelines. It’s intended to support technical literature supplied by the belt manufacturer or its agents.

Step 1
Follow your company’s safety practices during the installation. Use proper lockout/tag-out and personal protective equipment.

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 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 led to premature failure. Look for fabric wear on the sidewalls, cracking and oily surfaces. If you find any of these, do not install new V-belts until the root cause of the problem is identified and corrected.

Step 5
Clean the sheaves of all foreign matter with a stiff brush that includes 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 the V-belt’s outer skin and systematically destroy the belt.

Step 6
Using “go/no go” slip gauges obtained from a belt manufacturer, determine the condition of the V-groove in the sheave. This accurately determines if the walls of the V-groove have been subjected to excessive forces caused by improper tension. Such forces cause slippage and poor alignment between the driver and the driven shafting.

Step 7
If the sheaves don’t 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.

Never mix new and old belts, regardless of the “new” look of the old one. Always install belts in matched sets. This ensures all replacement belts are exactly alike. Never mix belts from different manufacturers because they have different stretch characteristics, coefficients of friction and cross-sectional areas. If the belts aren’t the same length, they won’t carry the same amount of load. This causes overloading and wear in some belts and shortens the life of the belt drive.

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

Check the total indicated run-out (TIR) of both the driver and driven shafts. These should be within plus or minus 0.003 inches. If the run-out reading exceeds this value, the shaft(s) must be straightened. Also carry out this check on the outer rim of each sheave, as it is common to find the shaft hole in the hub drilled off-center, causing damaging eccentricity. Eccentricity causes the belts to slacken off at the 3 o’clock position and 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 inches.

Check sheave alignment by placing a straightedge or a tightly drawn cord across the sheave faces so it touches all four points of contact. 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 aggravates the misalignment. When this is the case, align the Vs with each other. 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 and correctly align the grooves in the sheave.

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

Never install the belts by prying them onto the sheaves with a screwdriver or any other forcible method. This damages the internal cords of the belts and could break off the rim of the sheave’s sidewalls. This would cause unbalance of the rotating components. The motor must 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 inches.

For example, if the distance between the centers of the driver shaft and the driven shaft is 64 inches, the belt deflection load is:
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 slipping belts will move 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. Repeat until all slack is taken out of the belts.

Step 13
Replace the safety guard before removing all lockout and tag-outs. The guard should be constructed from open mesh steel. This permits free passage of air across the belt area, thus cooling the belts and allowing 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 belts (refer to Step 12). Before checking the belt tension, ensure all lockout procedures are in place.

Step 16
Restart the unit. This is probably the most ignored task in belt installation, but it is very important. As such, it is worth repeating the following: After the machine has run for 48 hours, check the tension on the new belts and retighten to the correct mid-span deflection set point. Repeat this process until there is no stretch.

Belts that squeal during acceleration or when operating at full load usually have slippage. Never add a lubricant to the belts. Squealing indicates belts need to tightening. This extends the life of the belts and bearings immensely.

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

John C. Robertson is the maintenance reliability specialist for Strategic Work Systems, a consulting firm with offices in Mill Spring, N.C., and Greenville, S.C.

This article appeared in the August/September 1999 issue of MRO Today magazine. Copyright, 1999.