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View Full Version : Is there an alternative to using pillow blocks?



djbutter22
05-18-2015, 09:44 AM
I'm currently using 2 pillow blocks side by side to hold my drive shaft. The pillow blocks are constantly slipping out of alignment one way or the other. What I'm wondering is, are you guys using anything else to mount your shafts? 5947

jwatte
05-18-2015, 12:11 PM
If they are slipping, then your mount is not strong enough.
Perhaps attempt to machine some grooves and edges that are exactly the size of the pillow block, so that there's something to hold it in place?

There are lots of different kinds of couplings and mounts that can be used, and bearings pressed into machined cavities. There are also MIG welders for making things stick in place ;-)
It all depends on how hard you want to work for it.

tician
05-18-2015, 12:37 PM
There is also the option of mounting the chain sprocket and the track sprocket on a common hub (machined length of pipe or rod stock) with internal tapered pin bearings that ride on a non-rotating/passive axle (with or without suspension). It works quite well for all sorts of bikes, scooters, motorcycles, and for the non-powered wheels of cars, trucks, etc.

djbutter22
05-19-2015, 11:58 AM
Thanks,
We are going to see if we can weld the bearing to the housing, if that doesn't work, I think we'll weld a bearing in a piece of plate and try that.

jwatte
05-19-2015, 12:40 PM
I wouldn't weld a bearing. That will very likely ruin the bearing (and set the lubricating grease on fire.)
I thought the problem was that the pillow block moved, not that the bearing moved in the block.

If the problem is that the bearing moves in the pillow block, then you need a tighter-fitting pillow block and bearing.
Also, it is often the case that you'll heat the bearing so it expands, then use a press to push the bearing onto a shaft, and when it cools, it will be very strongly attached to the shaft.
Then push the bearing into the pillow block; again probably using press fit.
This requires very tight tolerances, and, most importantly, it must be very snug, and you will likely need a press to get enough force to actually put it on there.

I suppose another option to fix the bearing in the block would be a pillow block with a C-like housing that clamps with big screws? I haven't seen any of those, but I imagine they must exist.

djbutter22
05-22-2015, 09:49 AM
Thanks guys,
Every pillow block or mounted bearing I've seen has some sort of self aligning feature. I've bought 2 different brands and they both shift out of place when I tension the track.

jwatte
05-22-2015, 10:28 AM
Are you using a single bearing for cantilevering the axle? If so, you probably want to add a second bearing on that axle, a little bit out from the first one. (The bigger distane, the better!)
The torsional/twist forces on a single bearing are very strong, because the axle gives it a strong moment and the bearing edge is close to the center of rotation. Adding a second bearing a bit out moves the "center of rotation" a lot and thus the local torque is much less.

djbutter22
05-22-2015, 10:38 AM
Are you using a single bearing for cantilevering the axle? If so, you probably want to add a second bearing on that axle, a little bit out from the first one. (The bigger distane, the better!)
The torsional/twist forces on a single bearing are very strong, because the axle gives it a strong moment and the bearing edge is close to the center of rotation. Adding a second bearing a bit out moves the "center of rotation" a lot and thus the local torque is much less.

On my original post is a picture of the application. I have the 2 pillow blocks side by side and the wheel and sprocket are mounted to the shaft on the outside. I know that the pillow blocks are meant for the the 2 to be spaced and the the sprockets mounted to the center but this is not possible in my case

tician
05-22-2015, 10:40 AM
The image shows two bottom mounted pillow blocks on the frame holding the axle to the frame.

If 'self aligning' feature is oblong mounting holes of the pillow block that allows it to slide perpendicular to the shaft, then you simply need to machine into, or weld onto, the frame some sort of rigid stop/detent/protrusion to prevent the pillow blocks sliding as the track is tensioned. Bolts alone will not be enough to prevent the pillow block sliding in the oblong holes. Trying to weld cast iron is not easy/recommended, and many cheap pillow blocks have cast iron housings, so welding the pillow block direct to the frame is not likely to work.

djbutter22
05-22-2015, 10:57 AM
The image shows two bottom mounted pillow blocks on the frame holding the axle to the frame.

If 'self aligning' feature is oblong mounting holes of the pillow block that allows it to slide perpendicular to the shaft, then you simply need to machine into, or weld onto, the frame some sort of rigid stop/detent/protrusion to prevent the pillow blocks sliding as the track is tensioned. Bolts alone will not be enough to prevent the pillow block sliding in the oblong holes. Trying to weld cast iron is not easy/recommended, and many cheap pillow blocks have cast iron housings, so welding the pillow block direct to the frame is not likely to work.

Thanks tician, It is the bearings that are moving out of place inside the housing. When I spoke to the bearing people, they said it was common for them to shift but unlikely if I'm using 2.

tician
05-22-2015, 11:26 AM
So the bearings have a 'sort of spherical' outer surface on the outer race that permits the bearing assembly to rotate a little bit inside the pillow block housing to compensate for up to a couple degrees of axle misalignment, but the combination of load and pillow block mounting/placement is such that the bearings end up rotating too far inside the housing and seizing up?

djbutter22
05-22-2015, 12:03 PM
Exactly. Even without any payload, they are being pulled out of place by the track tensioning around the drive wheels. If I don't tension it to the level of failure, then when I add the payload they will fail.

tician
05-22-2015, 12:26 PM
If you cannot space the pillow blocks further apart to lessen the amount of deflection, then things will be difficult. One option is what I described above with the fixed shaft and free-spinning hub on preloaded tapered pin roller bearings (often called 'timken', after the major manufacturer).

Another option is to machine a single block of metal to hold the opposed pair of tapered pin roller bearings and mount that to the frame. It will require machining the shaft to have steps and threads that will permit the bearings to be preloaded so they do not self-destruct and ensure they run true (castellated nut with cotter pin to press on one bearing in housing and step in shaft to press on the other bearing).

Probably a few more methods, but none are immediately coming to mind.

djbutter22
05-24-2015, 09:42 PM
Ok that is exactly what I needed to hear. I'm going to rebuild the frame and add some plate in the middle to mount the pillow blocks to. This will spread them about 12-15" apart from each other. The guy I was buying the bearings from told me that as long as I had 2 , they wouldn't slip so I mounted them next to each other. I will report back with success or failure

jwatte
05-25-2015, 12:32 AM
Tician: Why the castellated nut with cotter pin, instead of a shaft collar?

tician
05-25-2015, 03:06 AM
Tician: Why the castellated nut with cotter pin, instead of a shaft collar?
Partly because I'm paranoid, but mostly: if you're going to the trouble of machining a housing for taper roller bearings that require quite a bit of constant preload to not self-destruct, why not secure it proper? Stepped shaft, castellated nut and cotter pin means you can torque it down to ensure full seating of the bearings and races, back off the nut, then torque down again to the required preload; all without worrying about losing preload because the collar slipped or you didn't compress the bearings hard enough before tightening the collar.

Since the shaft is not transmitting power, the 'best' way would be a fixed shaft (preferably short as possible and machined from/into larger assembly attached to frame/suspension) and free-spinning hub with the drive and track sprockets. No continuous rotation of the 'too small diameter' and/or excessively cantilevered shaft means no (or at least far less regular) cyclical tension/compression loading that will often cause rapid fatigue failure like happened with Mule (http://forums.trossenrobotics.com/showthread.php?4314-More-Mule-then-Rover/page12) (which had it all: too small diameter for supporting bot's weight, too long/cantilevered, and transmitting power); just the usual shock stresses from the terrain and you can easily make the shaft/assembly larger/stronger to compensate and/or add suspension and/or just go slower.

jwatte
05-25-2015, 01:10 PM
bearings that require quite a bit of constant preload to not self-destruct

I see -- I read up some more on the tapered-pin bearings, and now understand why they need pre-load. (Being a mechanics n00b, I've just used ball bearings pressed into machined blocks before.) Thanks!

Sounds like a dead shaft would be ideal for djbutter22, if he has access to the necessary resources.

tician
05-25-2015, 01:30 PM
Even ball bearings can require a bit of preload for high precision operation. If you have any easily rebuildable stepper motors (NEMA sizes, not pancakes), you should usually find a wave washer, or similar device, at one end of the motor pressing on the bearings to ensure the bearings always run true. Remove the washer and that tiny bit of play in the bearings will often be enough to let the magnets in the rotor stick to the stator and prevent proper operation.