An effectively implemented, comprehensive preventive and predictive maintenance program for a parallel-drive system will reduce downtime, bring down costs, and create a safer working environment.
Parallel-shaft drive systems include v-belts, synchronous belt drives, conveyor systems, and chain drives. These power transmission systems are used for transmitting power and in some cases conveying product. Optimum performance and maximum service life can be achieved if the systems are correctly installed and properly maintained. All aspects of the inspection, installation, and maintenance process should be measured and documented in an organized and easily accessible manner.
The pie chart shown in Figure 1 depicts common problems in a parallel shaft system; individual slices’ share at a given facility will vary depending on different factors the facility faces. In any event, correct design, proper installation, and proper maintenance will have a significant positive effect on the drive’s life and performance.
Before troubleshooting the drive, all safety procedures should be followed, and a lock-out/tag-out verification start process must be performed. (No exceptions!) This process includes but is not limited to the following:
- Electrically de-energizing the system
- Load leveling or mechanically neutralizing the drive (a shift in load, pressure changes within the system, or the mechanical inertia present may cause things to move)
- Ensuring that all rotating components are properly guarded to prevent access when rotating
- Wearing all required PPE – work smart and safe!
A useful acronym for troubleshooting parallel systems is A.C.T., which stands for Alignment, Condition of the equipment, and Tension. These are the three most important considerations for installing and maintaining parallel shaft drives.
Alignment is needed for the drive to be able to run trouble-free. Misalignment of the sheaves, pulleys, and sprockets can cause vibration, premature wear of components, and additional loading on bearings. The alignment process of the sheaves or sprockets can be accomplished using levels, straight edges, string, and/or laser systems.
There are two types of misalignment: angular and parallel (offset). Parallel misalignment occurs when the driver and driven shafts are parallel, but the driven components, such as a sheaves, lie on two different planes. If the shafts are not parallel, then the drive has angular misalignment. There are two forms of angular misalignment: vertical and horizontal (Figure 2).
The easiest way to correct parallel misalignment is to shift the driver, which is usually an electric motor, or to change the placement of the rotating components on their respective shafts.
Angular vertical misalignment can be corrected by inserting or removing shims from under the front or back mounting pads of one of the machines.
Angular horizontal misalignment can be corrected by shifting, preferably with pusher bolts, the driver’s position so that the shaft centerlines are parallel.
A general guideline for v-belt drives is that they should have less than ½ degree of angularity and offset/parallel of 1/10” per foot of center distance. All synchronous belts and chain drives require alignment tolerances of half of that.
Offset is easily measured by placing a straightedge across the face of rotating components; any gap is measured with a feeler gauge. Angularity can be measured by laying a straightedge across the face of the two components – and ideally having at least three points of contact. Measure the gaps with a feeler gauge and use the following formula: 57.3 × (big gap X minus small gap Y) divided by the distance between the measured gaps, which is usually close to the diameter.
Condition of the components
The drive’s foundation must be solid. Guards should be up to industry standards. The belts or chain should not be worn, glazed, or cracked. Avoid the use of belt dressing if at all possible – if you need it, there are problems with your drive. The sheaves should not be worn more than 1/32”; this can be checked with a go/no-go gauge called a groove gauge (see Figure 3). Sprockets should be changed when noticeable wear occurs in the form of rounding or shark fining of the teeth. Never use a pry bar or a screwdriver to install belts. Their use may damage the tensile members and rotating components.
The amount of TIR (total indicator runout) should be measured radially and axially and should not exceed .001” per inch of diameter. The radial TIR is a measure of roundness; axial TIR is a measure of wobble or side-to-side movement of the components when rotated. It results from not properly tightening the fasteners of the tapered bushing used to mount components. The fasteners should be alternately and gradually tightened to the recommended specifications using a torque wrench. An air wrench will cause uneven draw-up, creating a wobble in the sheave as it rotates. It may even burst spoke-type sheaves, especially if an anti-seize compound is applied to mating surfaces. It’s probably best if anti-seize material is left out of the equation entirely when installing bushed sheaves and sprockets.
Tension is vital to all belting and chain drives. If there’s too little tension, slip, noise, heat, and vibration may occur. If there’s too much, the life of the belt, chain, and supporting bearings will be dramatically diminished. Several methods can be used to properly tension a v-belt or chain – none of these involves the thumb.
For the roller chain, the amount of measured deflection at the midpoint between shafts is used to determine tension, and it works out to approximately ½” for every foot of center distance on horizontally mounted drives and half of that for vertical drives.
With v-belts, you can measure belt elongation, calculate the deflection forces, use a sonic tester, or use a scale-type tension tester. Of these, the scale-type tension tester is the most affordable and the easiest to use (see Figure 4). The two variables when tightening v-belts are how far of a deflection and how much force. Belt manufacturers publish instructions for the proper use of the scale along with tables listing the amount of deflection at midpoint in pounds based on 1/64” per foot of span (center distance).
Introduce and perform A.C.T. procedures on your parallel drive systems. It’s all about the life of the components, and the better you maintain the drive, the longer it will operate efficiently and trouble-free. And remember… safety first!