A little help please

B

Bill Gruby

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There are two types of square broaches, one is the standard square the other the full square. The standard square in .375 has a .390 pilot. This makes a hole bigger that the square itself. The full square has a .375 pilot and leaves a square only. My question is this, I have to broach square holes in the crank cheeks. They call for the standard square on the prints. Will it be strong enough to handle the torque at 9500 RPM. The crank cheeks are 4140 and the shafting is 316 SS ------ "Billy G"

broach-shape-reference.png
 
I have to admit ignorance of your project... what size square hole, and how much torque are we talking about? The speed isn't likely to make a lot of difference, but that high a speed is likely to want a pretty good fit to keep it from rattling & buzzing like crazy.
 
How much horsepower is going though the shaft of the broached hole? Also, how thick is the plate that the broached hole goes through (that is, what's the depth of the hole)?
 
Ray --- HP is 20+ the 4140 0s .250 thick at the broached holes. Broach is .375 square. I am probably overthinking this but there will be a pretty good amount of money and time going into the radial before completion. It would be a shame to lose it because I overlooked something like this. ------ "Billy G"
 
OK, I can do these kinds of calculations. I've done them before but I need to look tonight at the Machinist Handbook for the formulas to refresh my memory. BTW: The calculations are in the section where they talk about keyways. I will also look at the the plans of the project to make sure I have the right understanding of the situation.

At the moment, I'm fixing and threading two boat prop shafts and they need to be done by tomorrow AM. Welding is done but they need to cool down before I straighten and bring them to spec. Sit tight.

Ray

Bill Gruby said:
Ray --- HP is 20+ the 4140 0s .250 thick at the broached holes. Broach is .375 square. I am probably overthinking this but there will be a pretty good amount of money and time going into the radial before completion. It would be a shame to lose it because I overlooked something like this. ------ "Billy G"
Click to expand...
 
I think you'll be ok with the standard. Using this calculator, (http://www.1728.org/mtrtrq.htm) I get, I get the torque on that part to be just under 16ft-lb (assuming 30hp, 10k RPM).

Doing some back-of the envelope calculations (worst case). Lets assume that the shaft is in bad shape, and all of the torque is being transmitted by one corner of the square, and that the contact area is only 1/16" wide. We'll also assume that our radius is about 3/16". So, the force on that 1/16" wide would be

Torque = force * distance

so

force = Torque/distance

which gives me

16ft-lb/0.1875in*12in/1ft = 1024lb

You say the material is 0.25" thick, so the contact area is 0.25"*0.0625" = 0.016 in^2.

Pressure on that area would be 1024lb/0.016in^2 = 65536psi.

Yield strength for 4140 is listed as 65ksi for annealed, and 125ksi normalized. That's close, way closer than I would normally want to see, but then again, we have looked at the _absolute_ worst case.

I'll see if I can come up with a better model for this, all my books are at home, and this is just an off the cuff guess. Regardless, I think the standard would work for you. The corners handle most of the torque anyway (if you think about it, the force is tangent to the flats at the middle, which basically means they support no torque, other than what they can support by friction). For me, I would be more concerned with getting a tight fit. If it loosens up and you get banging, the stresses will go way up.
 
First solution -- Go with the standard square and use #3/0 Taper Pins to hold everything in place. The Taper Pins will take the brunt of the forces. ------ "Billy G"
 
Bill,

According to CAD, the contact side is 0.134" (see below). Only 4 sides would be transmitting force so you'd have a total effective contact area of 4 x 0.134 x 0.25 sqin.

View attachment Bills Part.bmp

DMS's method looks good, we just need to re-calculate with the new value for effective contact area which I'll do in a moment when I have access to my usual calculator. Out in the garage right now, filthy beyond belief and calculator is up in the bedroom in my briefcase. I use what's called RPN calculators and I can't add 2 numbers on normal algebraic-type calculators.


EDIT: We also need to get the strength of the stainless... I'll look it up.


Ray
 
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