New Vibezone

[ Home | Contents | Post | Reply | Next | Previous | Up ]

Key length calculator

From: Bill Schmitt
Remote Name: 65.196.13.9

Comments

When considering unbalance in a keyed shaft application, how does everybody calculate what length key stock they need to install to keep things in balance? We have run into some apparent problems in the past with unbalance, and we traced it back to improper length key. We have a lot of high speed fans, that are all belted "speed up" configurations. All of our fan shafts are custom made, with key slots that vary in length. For instance, on a 3" shaft, some key slots are 4" in length while other shafts of the same size have a key slot 7" in length (don't ask why). So how does one go about calculating the proper length key for any given application? In our situation, here's what happened. We switched speeds of some of our fans, and that required a smaller driven sheave, that in turn required a different taper lock bushing, which in turn used a 3/4 height (rectangular) key as compared to the full (square) key that we had been using. Conveniently enough, the new bushings came with a 3/4 key that was the exact length of the bushing. So the mechanics, without questioning, installed the new sheaves and bushings with the provided key. This left about 4-1/2" of unused key slot in the shaft (a void about 1/2" x 3/4" x 4-1/2"). These fan shafts turn 2350 RPM. Now all hell breaks loose because things are very much out of balance. When we first discovered we had this problem, we decided to base our calculation for a proper length key on a "one for one" total volume of the key slots vs. key stock. In other words, if we had a square (full) key, and the shaft had a 4" slot and the corresponding bushing had a 2" slot, we made the key 3" long. This is obviously not the best way to do it, because the excess key stock length (to offset the unused shaft key slot) is at a further distance from the center of rotation of the shaft. However, this was a quick logical shot in the dark to get us close, and made things much better, but not perfect. Since then, I have had some time and I've been trying to find an exact formula. I have yet to come up with such a formula, so for now I've added a correction factor to offset the "overshot" in our first formula. I don't know if we're doing it right, but here's how I do it now: (shaft slot length * shaft slot depth * key stock width) + (bushing slot length * bushing slot height * key stock width) / (key stock width * key stock height). The answer ends up being the length (inches) of the key stock that would equal the volume of the shaft key slot + the volume of the bushing key slot. I then take 95% of that number and use that as correct key length. (The 95% was an average (91%-98%) of all the different key combinations and shaft sizes and speeds that we see). So far this seems to work out real good, but I know it's still not perfect. What does everybody think, am I splitting hairs to get it any more precise then that? Is there some kind of formula already out there that I don't know about?

Last changed: October 16, 2007