Industrial Motors

Follow these simple steps to stop sheave and belt breakage

Your millwrights can ensure sheave and belt reliability with these basic maintenance measures.

By Alan Luedeking

Good sheave alignment increases drive train efficiency and reduces premature wear or failure of belts, pulleys and bearings. You can achieve this state of high alignment two ways - the common, but labor-intensive, string-and-straightedge method, or by laser. The latter approach is now well established in the sheave alignment field, and has proved to reduce downtime and labor while achieving much higher accuracy. This results in significant savings and increased production uptime.

A V-belt drive is an efficient power transmission method exhibiting efficiencies from 90% to 98%, and has traditionally been thought of as forgiving. However, proper alignment and belt tension are critical variables that can make a difference in mean time between repairs (MTBR). Good drive maintenance involves a number of steps. Roughly half involve aligning the drive - removing belts, inspecting components for wear and damage, checking for bent shafts, correcting sheave runout, installing belts, alignment and tensioning the belts.

Your foremost concern always should be safety. Never allow loose clothing or long hair anywhere near belt-driven machinery, and ensure that the equipment is locked out and tagged out.

First, seek the root cause

Before beginning the sheave alignment, try to determine the cause of the belt or sheave failure and correct it to prevent a recurrence. The root cause of the failure might be associated with poor drive maintenance (improper belt tension, poor sheave alignment), environmental factors (ultraviolet degradation, harsh temperature fluctuations), improper installation (wrong belts/sheaves, belts pried on by force) or operating factors (overload, shock load).

Closely inspect the belts, each sheave and its grooves. Look (and feel) for cracks, chips and excessive groove wear (Figure 1). Check for proper contact between the belts and the sheaves. Correct any such mechanical and structural problems before proceeding with alignment to prevent premature or catastrophic failure.

Figure 1
Sheave friction can burn belts. Extraneous machine components can rub a belt raw. A belt that turns over in its sheave is doomed.

Change belts any time you detect undue wear. Inspect the used belt for any noticeable cracking, gouges or crumbling, and watch for glazing, which indicates belt slippage. The belt’s wear pattern can be a good indication of the type of misalignment or other problem that might be in play (Figure 3). Removing belts only requires slackening the tension by moving one sheave toward the opposing sheave.

Bring in the sheaves

The next step is to measure sheave run-out, which comes in two varieties: rim (radial) and face (axial). Both must be within tolerances before you can perform successful final alignment corrections.

Use the sheave or machine manufacturer’s tolerance recommendations. Otherwise, tolerance for radial or rim runout on high-speed sheaves (1,800 rpm and greater) shouldn’t exceed 5 mils total indicated reading (TIR), and this value can be increased to perhaps 10 mils on slower sheaves. The tolerance for axial or face runout shouldn’t exceed 0.5 mil per inch of sheave diameter (TIR) for high-speed sheaves, and can be raised to about 1 mil per inch for slower sheaves.

Check radial runout first. If it’s unsatisfactory, check the shaft runout. If that’s excessive, the shaft might be bent. If so, replace the shaft and check radial runout on the sheave again. If no shaft runout is detected, install a new sheave. 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 both runout values are within tolerance, install new belts. 

Figure 2

When multiple-belt systems need replacements, always use matched sets, not a one-by-one retrofit.

 

New belts must be the correct length, material of construction and pitch angle. Never force a belt onto a sheave. Doing so either will damage the tensile members in the belt’s core or damage the sheave itself. When aligning a multiple-belt drive, replace the belts as a set, not singly. New and used belts that are otherwise identical have greatly different operating and tensile characteristics (Figure 2). Also, never mix and match belts. Instead, only use belts from the same manufacturer, and preferably in a factory-matched set.

Store belts carefully

While you’re getting new belts, make sure your maintenance storeroom knows that belts are to be stored in a cool, dry place with no exposure to direct sunlight or near any source of heat, including HVAC systems.

Don’t hang belts from a single peg; this can damage the tensile members and distort the belt over time. Hang them from two pegs or, better yet, stack them flat on shelves. Coil the long ones, and don’t make your piles so big or heavy that they distort the bottommost belts.

Place the new belt(s) into the sheave grooves, reposition the sheaves to rough alignment, and check that the belts are properly seated within the grooves.

Laser versus string

Misalignment consists of three types: vertical angularity (twist), horizontal angularity and axial offset, all of which can be present in any combination and severity (Figure 3). The most common alignment method is the straightedge and string method. It requires that both straightedge and string be able to touch each sheave at two diametrically opposite points simultaneously (a total of four contact points) (Figure 4). Rotate the sheaves half a turn and recheck. Because a string can bend around corners, you can’t easily differentiate between offset and horizontal angle when only three-point contact is made. Nor will a straightedge or string detect twist angle under certain conditions. The approach is labor- and time-intensive.

A fan-type laser overcomes these problems (Figure 5). One such system mounts magnetically to the face of sheaves as small as 2.5 in. in diameter and projects a laser fan line onto three targets magnetically attached on the other pulley(s). Alignment requires nothing more than ensuring that the projected laser line cuts across the center of each target simultaneously. With additional targets, other pulleys in the drive (such as inside or outside idlers) can be aligned simultaneously.

Green lasers provide greatly increased visibility that’s ideal for bright-light conditions, especially outdoors on a sunny day, where red laser light can be harder to see. Whatever you use, though, in the name of safety, ascertain the laser class these systems use. The best units feature a Class II laser, which requires no special precautions beyond not staring into the beam.

Figure 3
There are three types of misalignment that can be present in any combination.

Apply the tools

Always correct vertical (or twist) angle first by shimming the driver. Then, correct horizontal angularity by moving the driver using lateral jackscrews. Finally, correct offset by moving the driver with axial jackscrews, or by repositioning one of the pulleys on its shaft, being careful not to cock the pulley in so doing.

Because performing one alignment correction almost invariably affects the other alignment conditions, the process might have to be repeated several times.
This is where the fan-line laser really pays off, because it allows you to monitor all three alignment conditions simultaneously as you work. This makes the job far easier and faster, and it increases alignment accuracy.

With the sheaves aligned properly, the last step is proper belt tensioning. Incorrect tension (as well as misalignment) adversely affect belt life and drive system efficiency.

Figure 4
Stretching a string across the sheave faces works, but takes time.

Use a spring scale to press down on the belt in the approximate center of its span (on the tight side). Note the force required to deflect the belt 1/64 inch per inch of span length. If you’re not sure of the belt span length, use the center-to-center distance between pulleys. You’ll be pretty close.


Tension the belt(s) until the deflection force equals the belt manufacturer’s maximum recommended values for the specific belts you’re using. Also, make certain this force doesn’t exceed the machinery’s design loads. The individual force values for multiple belt applications should fall within 10% of each other. A matched set of belts and a precision alignment are essential to achieving this goal. It’s tricky moving the driver to slacken or tighten the belts without changing the alignment. Here again, a fan laser pulley alignment tool is invaluable, because you can monitor the three alignment variables for the sheave set while you adjust the tension.

Figure 5
A laser beam replaces the taut string and simplifies the alignment process.

Rubber hits the road

The final step is running the machine for about two hours to allow the belts to stretch and seat themselves properly in the grooves. Then, retension the belts to the recommended values for new belts. Now, run the machines at least 72 hours, but not more than 10 days, and retension again, this time to the manufacturer’s recommended force values for used belts.

Alan Luedeking is manager of technical support at Ludeca, Inc., in Doral, Fla. Contact him at alan@ludeca.com and (305) 591-8935.

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