A V-Belt drive is a very efficient method of power transmission (from 90 to 98%) and has traditionally been thought of as very forgiving; however, proper alignment and belt tension are extremely important and can make a huge difference in MTBR (Mean Time Between Repair). Good drive maintenance involves a number of steps, roughly half of which involve aligning the drive. These are, in order: Removing old belts, inspecting all components for wear and damage, checking for bent shafts and correcting sheave run-out, installing new belts, performing the alignment, and tensioning the belts.
Your foremost concern should always be safety. Never allow loose neckties or long hair anywhere near belt-driven machinery, and make certain all equipment is locked out and tagged out.
Prior to beginning a sheave alignment, it is a good idea to try to determine (if possible) the cause of your belt or sheave failure, and correct it to prevent unnecessary re-occurrence. The cause of failure could be associated with poor drive maintenance (improper belt tension, poor sheave alignment), environmental factors (sunlight, harsh temperature fluctuations), improper installation (wrong belts/sheaves, belts pried on by force), or operating factors (overload, shockload).
When embarking on belt drive maintenance, one important step, often forgotten, is to perform a close inspection of the belts and of each sheave and its grooves. This inspection should include looking (and feeling) for cracks, chips, or excessive groove wear, and checking for proper contact between the belts and the sheaves. Problems of this nature must be corrected before proceeding with anything else, to prevent premature or catastrophic failure.
Replacing belts only requires moving one sheave toward the opposing sheave, to slacken the belts. Replace a belt with a new belt of the correct length, material, and pitch angle. Never force a belt onto a sheave as this will damage the tensile member of the belt, or damage the sheave itself. If aligning a multiple-belt drive, replace all of the belts together, not singly, since new and used belts that are otherwise identical have greatly different operating and tensile characteristics. Only combine belts from the same manufacturer, and preferably use a factory-matched set. Inspect the removed belt for any noticeable defect (cracking, gouges or crumbling) and signs of slippage (glazing). Notice where the belt is worn. This may be a good indication of what type of misalignment or other problem might be in play. It is appropriate to change a belt anytime undue wear is detected. When replacing a belt make sure you replace it with one that has been properly stored. Belts should be stored in a cool, dry place with no exposure to direct sunlight or heater drafts. Do not hang belts from a single peg; this may damage the tensile member and distort the belt over time. Preferably hang them from two pegs, or better yet pile them on shelves. Coil long belts, and don't make the piles too big or heavy, to avoid distorting the bottommost belts.
The next step of the process is to measure sheave run-out. There are two types of run-out: rim (radial) and face (axial), both of which must meet tolerance prior to actually performing final alignment corrections.
The tolerance for radial or rim run-out on high speed sheaves (1800 R.P.M. and higher) should not exceed 5 mils total indicated reading (T.I.R.) on average, and may be increased to up to 10 mils on slower sheaves. The tolerance for axial or face run-out should not exceed 0.5 mils per inch of sheave diameter (T.I.R.) for high speed sheaves, and may be increased to up to 1 mil per inch for slow sheaves. Always follow the sheave or machine manufacturer's tighter tolerance recommendations, if given. Start by checking for radial run-out. If unsatisfactory, check for shaft run-out. If excessive run-out is also present on the shaft, it may be bent. If so, you must replace the shaft and check radial run-out on the sheave again. If no run-out on the shaft is detected, replace the sheave instead. If the sheave is mounted on a tapered shaft bushing, remember to inspect and clean the bushing both inside and out to ensure proper seating. Next check for face (axial) runout (wobble) and if necessary correct it by repositioning the sheave on its shaft. Once run-outs are in tolerance, proceed to install new belts.
Place the new belts into the sheave grooves, reposition the sheaves to rough alignment, and check that the belts are properly seated within their grooves.
Now for the alignment. Misalignment consists of three types: Vertical angularity (twist), horizontal angularity, and axial offset, all of which can coexist in any combination.
There are several sheave alignment methods, the most common of which is the straightedge and string method, wherein these must touch each sheave at two diametrically opposite positions simultaneously (totaling four contact points). The sheaves should be rotated half a turn and checked again. Since a string can bend around corners, you cannot easily differentiate between offset and horizontal angle when only three-point contact is made; nor will a straightedge or a string detect twist angle under certain conditions. This method is also very labor and time intensive.
A fan-type laser such as that produced by the DotLine Laser overcomes all of these problems. The DotLine Laser mounts magnetically to the face of sheaves as small as 2.5" diameter and projects a laser fan line onto three targets magnetically attached on the other pulley(s). All you need to do is ensure that the laser line lines up with the unit's targets. Using additional targets, other pulleys in the drive (such as inside or outside idlers) can all be aligned simultaneously.
Always correct vertical or twist angle first by shimming the driver, then correct horizontal angularity by moving the driver with lateral jackscrews, and lastly correct offset by moving the driver with axial jackscrews, or by repositioning one of the pulleys on its shaft. (Be careful not to cock the pulley in so doing!) Since performing one alignment correction almost invariably affects the other alignment conditions, this process may have to be repeated several times. This is where a fan-line laser like the DotLine Laser really pays off, since it allows you to monitor all three alignment conditions simultaneously, as well as greatly increasing the accuracy of the alignment. This makes the job far easier and faster.
The last step, after the sheaves have been aligned, is the proper tensioning of the belts. Incorrect tension (as well as misalignment) will adversely affect the life of the belts and the efficiency of the drive as a whole. Using a spring scale, press down on the belt in the approximate center of its span (on the tight side), to deflect the belt 1/64" per inch of span length and observe the force required to do so. If you are not sure of the belt span length you may also use the center-to-center distance of the pulleys, which will be similar. Tension the belts until the force required for this deflection equals the belt manufacturer's maximum recommended force values for the specific belts you are using. Also make certain this force does not exceed the machinery's design loads. The force values for all belts should fall within 10% of each other. Using a matched set of belts and having a good alignment are essential in achieving this goal. It is tricky to move the driver to slacken or tighten the belts without changing the alignment! Here again the DotLine Laser Pulley Alignment Tool is invaluable, since all three of the alignment parameters can be monitored simultaneously for all sheaves while adjusting the tension.
The final step is to run the machines for about two hours to allow the belts to stretch and seat themselves properly in the grooves. The belts must then be re-tensioned to the recommended values. Now run the machines at least 72 hours but not more than 10 days and re-tension once again, this time to the manufacturer's recommended force values for used belts.
