Hub interference fit on a keyed shaft
Posted 12 March 2016 - 08:27 AM
It seems like in our industry most driveline components are splined, however there is the odd time you run into a keyed shaft especially on homemade things like drop boxes.
I've heard some different opinions on running an interference fit where the hub slides into the keyed shaft. Most people understand a key will hold a lot of torque and I agree, but I did run into a bit different falure the other day that i found fairly unusual
This is the main input shaft off a rotary screw compressor, it's driven by a 3406 CAT engine, in this application it's common practice to install the driven hub with an interference fit, the manufacturer in this example did not and basically let the key do all the work.
This is what the new installed shaft and hub look like
And this is what happened to the old shaft, by the looks of it the driven side of the key way work hardend and eventually started to crack, the entire shaft was so brittle in the spot where the hub was mounted was all full of cracks. the weird part was the key it self was in perfect shape,
Posted 12 March 2016 - 09:51 AM
All of the big crushers in the oilsands use a shrink disc coupling, which is essentially a n interference fit as well.
96' Ranger: kings, EMF parts, 800hp SBF
Posted 13 March 2016 - 07:53 AM
Thats a great failure. I've actually had similar happen and it was because my key groove was a c hair bigger than the than the key stock and best I could figure was it acted like a jack hammer on the shaft. When the pump sees pressure the torque would drive the key to the one lip and on de accelertation/ slow down/whatever it would shift to the opposite side like a worn u-joint. The hub holds the shaft together and gets the crap pounded out of it over a period of use. As it fractures the pieces have no where to go so the key just keeps hammering the parts until it makes it's way all the way around or until you see movement between the shaft and hub. All I did was make a new shaft and a tighter key slot.
So what's the correct fit? .001 tolerance and use heat to enlarge the bore for assembly? Puller for removal? The other thing that causes failures is improper key slot depth. I can't remember but I think the key is supposed to be 75% or more into the shaft. Not sure if that's a strength thing or difficulty of broaching the key groove into the hub. Usually the broach comes with the appropriate shims for ultimate depth if a company uses a shaper / Shafer? they can go to deep. That one looks a little light on the shaft and deep on the hub I think.
This is actually more commonly used than you would think. I work with this on hydraulic pumps all the time as well as pivot irrigation pumps, generators etc... I still haven't fully figured out the tapered shaft key way combo
Posted 13 March 2016 - 08:04 AM
There are three main types of keys: Sunk, Saddle, and Tangent, Round, and Spline keys.
Parallel keys are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over 6.5 in (170 mm) or when the wall thickness of the mating hub is an issue. Set screws often accompany parallel keys to lock the mating parts into place. The keyway is a longitudinal slot in both the shaft and mating part.
I'll see what I can dig up the last time I ran into this though I remember researching it for hours before I found what I wanted
Posted 13 March 2016 - 08:18 AM
I could be way out to lunch but if it was me the key depth definitely would be what I would be looking at for cause of failure. That and the fit of the key. When I get failures like this I take days going over them. People slam them back together blame it on whatever and carry on. Only for a failure a few years down the road. Probably why I drive my customers nuts they just want it running right away. A few years ago we had a pump throwing shafts. We made bigger ones up to 1450 u-joints which also sheared in half . I don't know much about water pumps but I asked if it was possible that there was a surge that caused the pump to suck itself together. I was told no. 4 drive shafts later and a split 3 foot diameter impeller form italy we pulled the pump apart and the key was machined wrong as well as a none locking nut (that was supposed to be that had backed off) allowing the impellar to suck itself into the housing trying to stall out the 3 or 400 horse diesel. I coulda saved two months down time and a ton of money but the pump company said it wasn't possible, the owner said the same thing and so did my shaft guy.
Posted 13 March 2016 - 08:18 AM
The other thing that I've seen that was pretty interesting is setups where someone has forgotten to install the key but with the proper interference fit there hasn't been any movement between the hub and the shaft, goes to show how much load is removed from the key with the proper fit.
Posted 14 March 2016 - 12:26 PM
a press fit usually is a press fit. As good as a weld no key needed. On a shaft the size in your picture .01 I think for a sweat fit. Odd they would use a key and a sweat. Guess there must be a shit ton of torque.
Posted 14 March 2016 - 03:26 PM
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