Sharing the load- friction and dowel pins
Sharing the load- friction and dowel pins
(OP)
I'm going to bolt a bearing block to a gear housing. The block will see a thrust load. I can't provide enough bolts to generate the clamp load needed to ensure that friction between the block and housing will react the thrust load. So I'm thinking of adding some dowel pins to help. The dowel pins will provide some shearing reaction force. I'm struggling with the load sharing between the two. I'm afraid the friction will react all of the load until there is slippage and then the pins will react all the load. When that happens the bearing block will have moved some amount. Does anyone have any experience with joints like these? The bearing block and housing are both aluminum.





RE: Sharing the load- friction and dowel pins
Redesign is cheaper than repair.
Paul Ostand
www.ostand.com
RE: Sharing the load- friction and dowel pins
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Sharing the load- friction and dowel pins
RE: Sharing the load- friction and dowel pins
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Sharing the load- friction and dowel pins
Dave Hyman
iRobot Corp
www.irobot.com
RE: Sharing the load- friction and dowel pins
RE: Sharing the load- friction and dowel pins
Once the block slips, how can you reliably estimate how much "kinetic" friction is present? The answer is that you can't. Therefore, the safest design is to provide a fixed shoulder to resist the block movement. In our case, we welded stop blocks in place, designed to withstand the entire side load.
Cheers,
CanuckMiner
RE: Sharing the load- friction and dowel pins
If so can you work the design to incorperate a nut?
If you have a nut and bolt arangement and are using grade 8 you might see what an "L-9" nut and bolt will give you for clamping force.
RE: Sharing the load- friction and dowel pins
The bolts themselves are always checked for the ultimate condition of friction failure and transfer by shearing in structural connections. THe pins can only add to this connection.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Sharing the load- friction and dowel pins
As all the variables are unseen I can only suggest that you mill a tight fixed pocket to install the block into. That way, if I understand the setup, the bolts would have to break or the block shear in half for movement to occur.
Or mill a looser pocket and use shims and hope they stay in place.
_______________________________________
Feeling frisky.........
www.tailofthedragon.com
RE: Sharing the load- friction and dowel pins
In addition to the bolts and dowels would it be possible to fit a keyway to help take the shear load.
The other concern I have is that you do not give any details of the block or the position of the thrust relative
to the mounting bolts.
If the thrust is offset from the mounting bolts then in addition to the shear load you might be faced with bending loads as well.
Perphaps you could provide more detail for us to assist you.
Regards
Desertfox
RE: Sharing the load- friction and dowel pins
There's not enough room for all the bolts I would need to get the friction nor would increasing the grade of the bolt be enough. I can't allow the bearing block to move as it has to locate gears on a shaft. Bending loads at the joint will occur but I have a handle on that.
Milling pockets or shimming or using stop blocks is an option at this point. Of course any way I do it will have to be very stiff to be able to share the load with the friction joint. I can't think of anything more stiff than the friction load path.
RE: Sharing the load- friction and dowel pins
Properly done, this arrangment works great, and there are hundreds if not thousands of successful machines out there using it, many of them in service for decades.
Typical practice is to ream the holes shallow in the factory, which allows some re-positioning (via re-reaming) in the field for final alignment.
The tapered pins are a little special, in that they have an internal thread at the large end of the pin to facilitate removal. The taper angle is selected to be self-locking, so no other retention means is necessary.
RE: Sharing the load- friction and dowel pins
RE: Sharing the load- friction and dowel pins
We still use bolts to hold the item in question (i.e. gear rack) on the pins. This is to eliminate the risk that a lateral force could knock the pins out.
RE: Sharing the load- friction and dowel pins
I'm having some difficulty imagining a bearing block that can't be bolted securely against thrust loads. A full sized feature via a machined block resting in a register on the main housing is the way to handle emergencies. Dowels Fitted to ensure alignment are fine, and even sized as duplicate system to handle loads. Varying loads mean Wiggling parts, even if only on a micro-wiggle scale. Wiggling parts restrained primarily by Dowels will fret and wear the mating faces, dowels, and dowel holes and someday be loose. Ask any Aircooled VW racer about their doweled crankshaft and flywheel. They will describe with pride that they doubled the number of crank/flywheel dowels from 4 to 8, using a fancy fixture to attempt to create 16 coaxial holes. http://www
If their motor was so powerful those holes get beat up, they fit oversize line-reamed dowels, or really stepped up and payed to have a taper OD fit added, or simply bought a crankshaft with a proper bolted flange. http:/
VW and Porsche abandoned dowels as a significant flywheel attachment detail in the 60s, as soon as their engines exceeded 100 HP or 4 cylinders.
Ask any Chevy or Ford or Dodge racer about what they had to do to upgrade their 6 or 8 bolted flywheel attachment, and they may have bought some better bolts for $20.
Is the block really a cap, with the bearing bore half in the block, and half in the main housing? http
Even if my calculator says the shear from the thrust can be handled by dowels, I keep picturing a significant moment being generated too, trying to pry the block off the housing, and the bolt preload had better be up to the task.
RE: Sharing the load- friction and dowel pins
Doesn't always apply, but in marginal situations it can make the critical difference.
RE: Sharing the load- friction and dowel pins
My aluminum block is similar to a pillow block bearing. I'm reacting loads from worm gearing - lot's of thrust on the shaft. Yes, there will be overturning moments on the joint but I can handle that with clamp load. I don't like the idea of dowel pins handling the reversing loads either(especially in aluminum).
White Tiger -
I don't know what you mean by "split taper seats". Can you expand on that a bit (or is there a sketch somewhere I could look at)?
RE: Sharing the load- friction and dowel pins
In effect, the tightened bolt becomes a defacto anchoring pin for the unit it's mounting.
Any comprehensive screw an bolt supplier will have them available.
RE: Sharing the load- friction and dowel pins
RE: Sharing the load- friction and dowel pins
I may have missed something but it seems no reply has been made to the suggestion about increasing friction between parts.
Alu vs. alu can achieve a maximum of 0.3 coefficient of friction; there are several commercial ways to increase it up to at least 0.5: silicium carbide deposition, for example, or sheets of carbide which can be 0.02 mm thick... Of course the point is if this kind of solution is acceptable from a cost point of view...
Regards
RE: Sharing the load- friction and dowel pins
RE: Sharing the load- friction and dowel pins
"My aluminum block is similar to a pillow block bearing."
Is the gear housing aluminum too? Regardless, the contact face can be a mighty weak link. Aluminum joints subject to bending or side loads (should) make their designers worry about shuffling. To fight shuffling and fretting elaborate interlocking serrated features are applied to resist shuffling. Straight serrations are typical. The onoly pictures I could find are for interesting circular serrations
http://ww