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Shoulder Bolts?
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Posted by: lhunziker

06/15/2010, 01:01:27

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Ok, so being a "greenhorn" out of school in Aer E, I've been doing some test engineering and one of the issues we've run across is the use/application of shoulder bolts. Basically, the idea is to have two parallel shoulder bolts for a pneumatic brake caliper. However, it is apparent after some time in operation, the calipers are not allowed to move freely on the shoulder bolts, likely due to non-parallel conformity between the two bolts.

My question is this - currently, the shoulder bolts are mounted with a counter bore into the mounting surface. However, on a different application with the same caliper, they are mounted with the shoulder face simply resting on a machined flat surface. These bolts have had no issues with parallel misalignment after long periods of operation.

Is there a governing spec for the tolerances allowed in the "squareness" of the shoulder face with respect to the threaded portions longitudinal axis?

What are proper applications of shoulder bolts (counter bore or none)?

Thanks in advance!








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: Shoulder Bolts?
: Shoulder Bolts? -- lhunziker
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Posted by: RWOLFEJR

06/15/2010, 10:02:36

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First thing runs through my head is there isn't enough clearance right out of the chute. Next I think... how many samples are we talking about? My thinking is if you don't need a Swiss watch don't build one. When an application can permit more error or misalignment then allow it.

In my past experiences with brakes and machinery clutches etc. sliding on pins and all... there has always been quite a bit of clearance at the pins and bushings. With all the crud that develops in a brake or clutch system, you would need to protect the slip members if you feel it's necessary to keep fits close or tight.

Some other things to look for...
Are the pins indeed out of parallel or is this an assumption?
What are the... as new condition... clearances in the two different designs?
Are the same materials used in both designs?
Is the load applied at the same point in both designs or does one apply the pressure further off center of the pins? (Could the sliding member be cocking in one design more than the other.)

Far as the application of stripper bolts goes there's no "rule" I'm aware of. The tensile strength will not change by sinking it into a c'bore but the shear strength will be greater. You need to look at your application and decide what makes the most sense. If you're going to sink the shoulder into a c'bore then yes the c'bore needs to be concentric to the threaded hole and the floor needs to be perpendicular to the thread. This really ought to be a non-issue with good machining practices. You might want to review the steps taken to machine the parts to see if an operation allows for misalignment. Example the parts are faced then removed from the machine and placed into another machine... jig... fixture or whatever. If the second operation is poking holes and the second set-up is off (or visa versa) you'll have issues. If you're going to run the shoulder into a c'bore it'd be best if your counter bore tool is cutting a couple / few thousandths big. You also need to be certain that the head of the bolt has plenty of clearance if it's in a bore. At least a thirty second of an inch.

Good luck,
Bob








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: Shoulder Bolts?
: Shoulder Bolts? -- lhunziker
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Posted by: jboggs

06/15/2010, 08:52:17

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First problem - Shoulder bolts are probably the most misused mechanical component. The fact that the thread geometry of the mating parts must necessarily include clearances leads to unpredictable and unrepeatable alignments. When one shoulder bolt is used as a pivot point or a single alignment guide this usually isn't a problem. But when used in pairs or more, then conflicts are inescapable and should be anticipated.

Second problem - The "shoulder" in shoulder bolts is a natural stress concentrator and is a likely fracture point. Any lateral loads on the bolt are magnified and amplified at that point. Your design should ensure that lateral loads are minimized.

I don't see how this is possible in a brake caliper since all the load is lateral. I would recommend that you find a way to use standard press-fit dowel pins with a properly machine alignment. There are other options two.

Third problem - you are possibly also committing the most common mistake in design of linear motion systems. I call it inadequate bearing spread. Let's call the length of contact of the caliper bushings on the shoulder bolts "L". Then lets call the distance between the two shoulder bolts "D". The smaller the L / D ratio, the more likely you will have binding.

The problem you are seeing might not be an issue of misalignment. It might be "unequal" motion on the two shoulder bolts leading to binding. In that case, longer contact bushings should be considered.








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Posted by: lhunziker

06/15/2010, 19:13:36

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Jboggs,

Haven't had time to think much on the actual design of the system (not my design, rather, just my inherited problem) yet, but I did check your L/D quantity, and for this application, L/D ~ 0.9-1.0. Granted, that is just a quantification, but what is the significance of being on either side of 1.0? is 1.0 an optimum, or could one suppose that the larger the L/D the better, even beyond 1.0?

I'm not sure what the spec is on our shoulder bolts, but I guess the rough numbers are braking torque of 400-600Nm with something near a 13mm? bolt. I'm not positive as I don't have the drawing handy, or the bolt, but in IPS it looks to be a 1/2 or 5/8" bolt (shoulder diameter). I think the brake rotor has a diameter of 14".

I'm not sure how much lateral load (I assume from the applied brake torque) this would transfer to that portion of the bolt. I imagine the guys who put this design together must have thought about the lateral loading at some point, but I haven't had a chance to discuss it with any depth. I'll investigate further and report back if I find anything.... interesting.

Thanks for all the insight!








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Posted by: Kelly Bramble

06/15/2010, 09:10:36

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Very good jboggs!!







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