# & size of bolts for disc brake flange...

[azonicbruce]

I need to design a flange for a disc brake rotor that will be used to stop an automobile shredder (the rotor and hammers). Here are the specs on the brake rotor/caliper that were given to us by our brake supplier:

Rotor Disc Material: St 52.3 (ASTM A738 equivalent)
Rotor Disc Diameter: 2.6m
Disc Thickness: 45mm
Disc Speed Start: 450 rpm
Disc Speed End: 0 rpm
Braking torque total: 141913 Nm
Braking time: 15 sec

The BCD for the flange I need to design will be 12.125", and will be the same thickness as the rotor.

I'm just wondering if there's a good reference for determining how many bolts I need for the flange, and how big they should be? We would probably be using Grade 8 bolts. Also, would it be better to have threaded holes in the flange, or to use through holes and attach the rotor using bolts and nuts?

Thanks

 

[jetmaker]

azonicbruce,

Go with thru bolts c/w nuts. Threaded parts are just a cry for fatigue related issues.

As for the sizing, you have the maximum torque, apply a suitable safety factor. Then assume a specific arm for the location of the bolts and calculate the total force to react the torque. Look at the bolt shear and bearing capabilities, and divid this number into the total force. Round up, and that's the number of bolts required.

jetmaker

 

[desertfox]

Hi azonicbruce

To calculate the mean shear stress in the bolts for your given torque proceed as follows:-

1. assume a bolt size

2. calculate force at bcd for the torque you have quoted
ie:-141913/((12.125"/2)*.0254)=921587.79N

3. assume a number of bolts say 20

4. divide force by number of bolts = 921587.79/20 = 46.1kN

5. workout the c.s.a area of the bolt and divide it into
the force calculated in step 4.

6. compare shear stress in grade of bolt to that calculated
and you need a good safety margin, max shear stress in a
bolt is approx 50% of max normal stress if you know the
yield stress of the bolt, half of it will be the max
shear stress (approx)so your calculated figure needs to
be well below the latter.

This is be no means a complete answer as there are many things to consider ie:- bolt hole diameters, fatigue if this thing is stopping and starting,bearing stress on the flange materials bolted together.This is not my normal area of work however I am surprised that you don't mention any keyways which is another way of resisting torque loading.

regards Desertfox

 

[azonicbruce]

Thanks guys.

OK, so let me see if I did this right:

SAE Grade 8 bolts are 150ksi ultimate tensile and 91ksi ultimate shear.

921587N ~ 207181 lbs

Choose 16 1-inch bolts:
(207,181/16)/[pi(.5)^2)]= 16.4ksi per bolt

If I choose 6 bolts (still 1"), it comes out to 43.9ksi per bolt, almost 2 times less than the 91ksi ultimate shear value.

Is this the same as having a factor of safety of 2? I forgot how/where the saftey factor gets applied to.

 

[desertfox]

Hi azonicbruce

Yeah your maths are okay. However if you use the bolts consider what clearence holes you use in the mating flange,I say this because all 16 bolts may not be in contact
with the flange wall which means the torque is carried by fewer bolts. Also I would use the proof or yield stress to
obtain my shear stress figure not the ultimate tensile value.

regards desertfox

 

[jetmaker]

azonicbruce,

You still need to look at the bearing capability of the plates/flanges. If they are really thin, or soft, then they will fail in bearing long before the bolts reach their shear capability.

Next, you want enough bolts spaced around the perimeter to avoid buckling/warping of the plates during the braking operation.

So, it might be better to use smaller dia bolts, but more of them.

Also, if this equipment poses a risk to safety or property, check your local codes, and if nothing else, use a factor of safety of 5.

Finally, as desrtfox suggests, you may want to consider the hole fit. If I was designing something like this, I would alternate the fastener type at holes. For example, use shear/dowel pins in tight/interference fit holes and tension bolts in clearance holes. The shear pins transfer the load by shear, and the tension bolts by friction between the plates.

jetmaker

 

[unclesyd]

There is a lot of merit in the post's of desertfox and jetmaker.
This is a rough application if it is anything like the local shredders. I haven't seen how the drive is configured. Their operation is chaotic, load, unload, and overload.

If the flange is at the end of the shaft I would use a snug fitting centering hub if possible. This will take a little load off the driving flange.

I would seriously think about using a shoulder bolt with a good (reamed hole).

I am hoping this brake will only be used for shortening the coast down or an E-stop.