It’s About Time we Treat Belt Driven Machines with More Respect
By John Lambert
Belt driven machines are the Rodney Dangerfield of the maintenance industry – “they don’t get no respect!.”
November 23, 2020
By John Lambert
Belt driven machines do need a little bit more TLC than they currently get. Two innovations will greatly improve your maintenance efforts on these highly efficient drives. Also, the general way in which we work with belt drives is a good example of what keeps reliability unachievable for many.
Some individuals/organizations really understand the value of doing this work right. Gary Burger wrote an article on improving the efficiency of belt drives. He says that he “expects to get three to five years from a belt that is running 24/7.” He gets that, and you can too. Keep in mind that this is not a roof fan that will spin forever, it is a working machine that needs sizable horsepower to turn it over.
The reason these drives do not get the respect they need is experience. Like many of you, I’ve seen belts pried off, and a new set pried on; large sheaves installed with air guns and never checked for run-out, so they vibrate in the axial plane; string used for alignment or a length of angle iron from the steel rack used as a straightedge for alignment. Air guns have been used to tension a belt so tight that the belt would not defect, even if you walked on it; belt dressing, which used a can at a time to try and stop the belt from squealing. All of these are not acceptable if you plan on doing the job right. The first time.
What is belt misalignment?
The number one reason why belts fail prematurely is heat. Heat dries the belt out, which leads to cracking and slippage and the end is not far off after that. The heat is from friction caused by misalignment. To correct misalignment, we must correct the offset and angel errors in both the horizontal and vertical planes.
• Angular misalignment in the horizontal plane. (Toe-out and toe-in)
• Parallel (offset) misalignment in the horizontal and vertical planes.
• Angular and parallel (offset) misalignment in both planes. This is what we are normally trying to `correct.
One of the reasons why it is difficult to correct this misalignment is because you are constantly correcting the alignment as you tension the belt. If you have ever worked with an adjustable motor base that has one central adjusting bolt allowing the motor to pivot as you tighten it, it is a challenge. These bases are lightweight, flexible, and cheap.
The inexpensiveness is why they are used but there is a hidden cost. You do a lot more maintenance work with this type of base so there is no savings. The frustration comes from the fact that you are unaware the bases are flexing until it’s too late. This is because it is not something you can see when using a straightedge to align the sheaves. You still must compensate or adjust the alignment as you tension the belt.
Innovations Effecting Belt Alignment
Two of the same style bases used to drive reasonably sized fans. They are taller than common bases and that is because the motor can pivot forward and back to allow for tensioning. The pivot point is square tubing that has a rubber insert, which creates resistance. This has an effect of spring loading the belt tension. It reduces the stress on the motor bearings because of over doing the tensioning. The base is solid and does not flex but allows for angular movement. Simple to install, but you still must check for soft foot or distorted footings as you normally would.
It separates the two critical components of the installation of the belts. The alignment and the belt tensioning. You align the sheaves in the normal way by moving and shimming the base. Then simply tilt the motor back to tension the belt. The tensioning will not affect the alignment as the motor comes straight back. The alignment is maintained and has no effect on the tensioning. It’s easy and efficient, provided you used the right tool to do the alignment.
With all advances made in the maintenance world, some are still using a string or straightedge to do alignment. This is still the common form of belt alignment today. Using a piece of string to align two sheaves should be left in the past. A laser should be used.
Why? When you measure with a laser you can measure the horizontal from three to nine o’clock and at the same time measure the vertical from 12 to six o’clock. To try and do all of this with a straightedge is exceedingly difficult, if not impossible, and that is where the saying “that’s close enough” comes in.
The Alignment Process
Begin with the vertical plane from 12 to six o’clock and adjust by adding or removing shim from the base. We do the vertical plane first with the shimming. If you started with the horizontal plane first, moving the motor side-to-side, and then did the shimming in the vertical plane, you would inadvertently disturb the horizontal alignment adjustments you already did. It’s always best beginning with the vertical, and shimming the motor as necessary.
Next, correct the horizontal plane (toe-out and toe-in). This will remove the angle from both planes. Lastly, adjust the offset sliding the sheave/pulley forward or back on the shaft. This is the same process that you can use with a visual/optical or digital laser system.
One trick used is placing the laser transmitter on the stationary machine, a fan for example. The laser beam will point to the motor. The beam will pass the motor and will be seen on a fixed item such as a pipe or a wall. If you mark where the beam is hitting and watch it as you apply the tension to the motor. If the beam moves off the mark it means the stationary machines base is flexing.
Laser systems are impressive. For me, the best feature is that you can create an as Found and an as left report in a PDF format. You can e-mail this report from the display or from your phone to the customer, boss, or planner, who can attach it to the machines history.
This is important as some belt drives can put production down. If you wanted to improve the asset reliability in your plant and decided to do some breakdown analysis after losing a belt, without the documentation, you would be guessing at what happened.
The current way of doing belt drive installations and the maintenance of them is an example of why asset reliability is un-achivable. That is because we use poor installation procedures. For example, a belt drive squeals at start-up. A work order is made to tighten the belt. Is the belt loose? Or is the sheave the wrong size? Many OEM machines have the minimum size sheave that are good when new, but when a little worn they slip. That is because there is limited belt rap on the small sheave. If this is the case, the repair is to replace the sheave with a larger size so that the belt has more area to grip. In my experience, what happens is the belt gets “re-tightened,” which is another way of saying it gets overtightened.
If tradesmen are still using string or a straightedge to align sheaves, the statement we are making is “close enough is OK.” The perception that a v-belt is a rubber band and can handle misalignment is wrong.
It is not just belt driven machines that are poorly installed. It is a fact that the biggest influence in a machine’s life expectancy is the installation. Therefore, the reliability of the asset is also affected. MRO
John Lambert is the President of BENCHMARK PDM. He can be reached at john@ benchmarkpdm.com.