FEM CONSTRAINT
FEM CONSTRAINT
(OP)
I'm trying to figure out how to constrain a wheel model. I'm only analyzing the wheel center of a 3-piece wheel. It is in NO WAY in contact with the tire. I want to know how I should constrain the hub face and the studs. I realize that the nuts on the studs will be torqued and there will be a compressive load at the lug holes. But I want to know, is a torque about the wheel axis transmitted soley between the hub and the wheel hub face? Or do the studs take some of this load as well.
Thanks!
John
Thanks!
John





RE: FEM CONSTRAINT
The worst case senario would be to have the wheel studs taking out all the torque. This is conservative but very reasonable when looking at ultimate load conditions as the friction forces between wheel and axle hub are likely going to be exceeded.
To model this problem as accurately as possible, I would look at a simple contact problem in which the wheel hub is in contact with the axle hub, resisting only inward loads. The studs would then handle any outward loading through tension. Furthermore, the stids would carry all shear loads due to vehicle weight and torque.
Hope this gives you a start.
jetmaker
RE: FEM CONSTRAINT
Does this seem right?
You answered my main question about whether the friction between the wheel and hub would be exceeded.
How should I go about calculating the compressive load due to the torque in the nuts?
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
I am not sure what a hubcetric wheel is, but to determine whether the shearing loads are taken by the studs or hub is simply a matter of tolerance. All the vehicles I've own have had large clearances between the wheel centerline and the axle hub. Also, they all used beveled nuts which nest into the holes in the wheel. So, the wheel load is transfered into the nuts and then through the studs. So the wheel never really contacts the studs directly. If this is the configuration you have, it looks like the load passes through the studs first.
Analyzer calls it correctly. Calculate the preload in each bolt based on nut/stud torque. If the idea is to add this preload into the analysis, apply it concentric to each stud over a small diameter about the stud hole in the wheel.
Have fun... and a good weekend.
jetmaker
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
1. Hub Piloted Wheels (spedtoe169 calls this hubcetric)
A single cone lock nut is used on each bolt, for single or dual wheels. Wheels for this type system have straight through stud holes without a spherical chamfer. In this design shear loads are taken by the hub.
2. Stud Piloted Wheels
Wheels of this type have a spherical countersink on each stud hole. In this design shear loads are taken by the studs.
In both designs studs will see tensile loads due to nut torque, etc loads.
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
Question for you... would you typically account for friction during ultimate loading??? What about for fatigue loading???
The policy at the company I work for is that friction is neglected for ultimate loads as the applied loads are often thought to exceed the static friction capability of a joint. I would agree that if the surface finish of the mating parts was sufficiently rough, and the preload high, it would be a reasonable assumption to consider friction effects.
spedtoe169,
There are many ways to calculate the preload on a bolt due to friction. One accepted method is detailed in MIL-S-8879(c). Shigley also specifies a method for calculating it.
Regards,
jetmaker
RE: FEM CONSTRAINT
Assume for the sake of argument that a conventional wheel is attached with five 1/2" studs each preloaded to 5000 lbf (a not unreasonable figure). Assuming a coefficient of friction of .2, it would take a transverse force of 5000 lbf before the bolts experienced any shear load whatsoever. And the studs would probably be subjected to bending loads as well as shear at that point.
In the case of hub piloted wheels, I thought the issue was whether the studs see shear loads due to torque, which again depends on friction.
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
Stud piloted Wheels used to gouge/break due to shear and bending loads on them , the stud hole size was close to the stud size that is why the industry (large truck) is getting away from that design.
In the case of hub piloted wheels the stud hole is so much bigger than stud that it is not practical for the studs to take shear loads under NORMAL loading and proper torque.
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
For the purposes of my analysis, I'm assuming that everything is torqued properly and won't come loose. The vehicle we're talking about weighs 2700lbs with driver and no fuel (it can carry about 14gal).
My company has no policies for anything of this sort. Part of my task is to develop such criteria.
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
Torques would be reacted by the same mixture of paths.
Incidentally typical peak shock loads at the wheel centre would be of the order of 3-5 times the wheel loading, on a car.
I have a feeling that you will need to model the hub and studs in order to investigate the propertion of the loads taken by the studs and by friction properly.
What I do know for sure is that the design of optimised steel wheels is quite tricky.
Cheers
Greg Locock
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
RE: FEM CONSTRAINT
This is at least roughly calculable. Say lateral acceleration is 1g, mass is 400 kg rolling radius is 600 stud pcd is 120, and worst case (for a 4 stud fixing) is that all the cornering load is taken in tension by one stud then it sees a force of 400*10*600/60, that's 40 kN
Now, I'm not at all sure what the preload is, but say the wheelstud has a thread with a root diameter of 10 mm, yield at 500, torqued to maybe half yield, that is a preload of 19 kN.
So, the cornering load is of the same order as the preload, ie it needs a much more careful analysis to get the right answer.
Cheers
Greg Locock
RE: FEM CONSTRAINT
Merry Christmas, which you guys usually celebrate on the beach don't you?
RE: FEM CONSTRAINT
Yeah, Chrimbo on the coast this year, 28 degrees C, lots of sunscreen.
Cheers
Greg Locock
RE: FEM CONSTRAINT