and Maintenance Can Prolong the Life of V-Belts
C. Robertson, maintenance reliability specialist
V-belts run longer and perform better if they are
given the proper care and attention during installation, and in particular,
during the following 48-hour running-in period.
This is a most critical time for V-belts, especially if they are to last for a
few years. During this run-in period, the initial stretch is taken out of the
belt. Also, the soft rubber surface of the belt's outer envelope is abraded
away, and the belt settles deeper in the groove of the sheave.
This causes the belt to run slack. At this point,
the slack on the new belts must be taken up to avoid considerable slippage,
frictional burning, and other irreparable damage. It is very important that the
belts are checked often over the first few days of operation and are adjusted
according to the correct tension until all signs of stretching have been
eliminated. This practice will eliminate early damage and promote longer belt
This article is intended to provide maintenance personnel with a standardized
procedure for correctly installing a V-belt and the sheaves in which they
operate. This, in turn, improves the mechanical efficiency of the motor and the
driven mechanical equipment by reducing wear on rotating mechanical components.
This procedure provides general guidelines for the operation and maintenance of
V-belt drives. It is intended to support any technical literature that may have
been supplied by the belt manufacturer or their agents.
Follow your company's safety work practices during the installation of the
V-belts, including personal protective equipment policies and lockout and
Remove the safety guard from the V-belt drive area.
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.
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.
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.
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.
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
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.
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
- 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
- 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.
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.
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
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.)
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.
Start the unit and allow the belts to seat themselves in the grooves of the
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.
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.
to Belts Reference Articles Index
© Copyright 2007
Maintenance Resources, Inc.
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