CTE of different shapes of 4130

[dnshift]

I have a cylinder that I am working on that has pretty tight tol. and I am questing the basic design. Picture a 8.0000 diameter x 6.000 solid double acting piston with a rod diameter of 6.000 in an 8.0005 cylinder with 6.0005 bearings. In order to reduce the hydraulic working area of the cylinder for resolution purposes, the band-aid design uses two rings at 8.000 x 6.0002 x 1 to attach to the rod at the piston.

My question...

Will the piston, rings and cylinder expand at the same rate and to the same relative dimensions when heated from 75 degrees F to 200 degrees F? I contend that they will not, and that the rings will change enough to cause possible damage to the cylinder walls.

And don't ask me why the tolerances are so tight, I have no idea, it seems crazy to start with to me.

Thanks in advance for your reply.

 

[MikeHalloran]

If everything is the same material, and the temperature changes slowly, it shouldn't bind. ... but gradients in temperature might cause some difficulty, given those clearances.



Mike Halloran
Pembroke Pines, FL, USA

 

[mrfailure]

Product form makes no difference; you will have the same. Per ASME B&PV Code Section D, the mean CTE for carbon and low alloy steels (including 4130) is 6.7X10-6 in/in/deg-F.

Aaron Tanzer

http://www.lehightesting.com

 

[dnshift]

Thanks. We had a failure and are trying to get some ideas as to why. Bonus question, if said rings were to have an interference fit to the rod, what would be a good number to grind them to? I am thinking .002-.003, but am concerned the rings might crack under the stress. Is there a way to calc that out?

Thanks again.

 

[MikeHalloran]

Machinery's Handbook has an equation to calculate the force required to assemble a press fit. I don't think it tells you the stress, but some engineering textbooks do, perhaps not as directly as you'd like.

Rule of thumb: .0001" interference per inch of diameter is a good start. Two thou is a fairly heavy press fit for steel even at 6.000 diameter.

If the piston and cylinder and end cap diameters, all of them, are not dead nuts concentric, and they pretty much can't be, that's another contributor to stress, and should be checked. The usual preventive measure for problems like that is a little extra clearance in one set of mating diameters.

Photos of the failure might get you additional inputs.






Mike Halloran
Pembroke Pines, FL, USA

 

[salmon2]

How about using hoop stress of a tube to calculate?

 

[ricklts]

Have you considered that Thermal Expansion is also a volumetric problem. Yes, all will expand at the same rate, but the larger pieces require longer periods of exposure to reach the upper equilibrium of expansion.

Something to think about....