February 22, 2023
Sheave Alignment: Aim Small, Miss Small
By Stan Riddle
A customer recently called saying his Belt Hog Sheave Alignment Tool was out of calibration. He stated that when he placed the Belt Hog lasers in the sheave grooves, they showed a slight amount of misalignment. However, when he rotated the lasers 180 degrees to check, there was a variance of 3mm (0.118”).
While this amount is small, I thought it should have been closer. So, I did a little math to see if I could figure out his problem.
- The distance between the Belt Hog lasers was 3 feet.
- The contact surface of the Belt Hog is 0.750” (the tops of the sheaves).
- The variance was 0.118” (3mm).
Across the 3-foot span, this 0.118” (3mm) variance is 0.183 degrees, or 0 degrees, 10.98 minutes.
Not very much, from the degree of precision necessary to align sheaves/belts, but not as close as I was expecting.
I did a little more math, to see how much variance would be required across 0.75” to cause a variance of 0.183 degrees.
0.0023” – that is a pretty small number!
So, I dug a little further.
I mounted my Belt Hog in a vise on a surface plate and used a dial indicator to measure any surface flaws on the magnetic rails (the parts that hold the Belt Hog onto the sheaves).
Mine were a little beat up, from using them. I found the worst “dent” was about 0.003”.
This is more than enough to cause a variance similar to what the customer was having. I stoned the surface three strokes with an oilstone and reduced the surface variations to less than 0.001” and I recommended the customer to do the same.
Back to the customer’s perceived problem. There were two separate things:
- The degree of repeatability: That was corrected by stoning the burrs from the contact surfaces of the Belt Hog in question.
- Expectation of what is an “in tolerance” sheave alignment: His expectation was zero. “Zero” would be difficult and is unnecessary. The typical sheave alignment tolerance for V-belts is 1/16” per foot between shaft centers.
NOTE: the typical alignment tolerance for V-belt sheaves is 0.5 degrees, per most sheave manufacturer’s recommendations, but that is hard to measure. We teach aligning them to 1/16” per foot (or less) between shaft centers.