Problem in design of a piston and cylinder for elevated temperature
Problem in design of a piston and cylinder for elevated temperature
(OP)
Hi friends,
I have a cylinder with inner dia 40mm, and outer 100mm. A piston has to slide in the cylinder to inject a molten metal in 600 centigrade degree. Both of the parts are made from H13 tool steel. I was considering about using H7/g6 tolerance between two parts. But the problem is that when these parts bring into those high temperatures they stick each other, preventing the piston from slinding.
I analyzed the two parts using ABAQUS (FEA software). Since the material used for the parts are the same (both H13 tool steel), the analysis shows no or a little change in tolerances between two parts in elevated temperatures. But in practice the problem remains, and the parts stick to each other. I was wondering if the material looses its hardness in those temperature that causes a severe wear in the first stages of sliding, and then the parts stick.
I am really confused.
More discussion about this problem can be found here: thread1103-250208: Elevated Temperature Tolerances!
I have a cylinder with inner dia 40mm, and outer 100mm. A piston has to slide in the cylinder to inject a molten metal in 600 centigrade degree. Both of the parts are made from H13 tool steel. I was considering about using H7/g6 tolerance between two parts. But the problem is that when these parts bring into those high temperatures they stick each other, preventing the piston from slinding.
I analyzed the two parts using ABAQUS (FEA software). Since the material used for the parts are the same (both H13 tool steel), the analysis shows no or a little change in tolerances between two parts in elevated temperatures. But in practice the problem remains, and the parts stick to each other. I was wondering if the material looses its hardness in those temperature that causes a severe wear in the first stages of sliding, and then the parts stick.
I am really confused.
More discussion about this problem can be found here: thread1103-250208: Elevated Temperature Tolerances!





RE: Problem in design of a piston and cylinder for elevated temperature
The problem is, in practice, it's unlikely you have the same temperature across the piston face and throughout the cylinder. I.e., the i.d. of the cylinder may be at 600 C, but the o.d. of the cylinder may be quite a bit less (due to heat transfer to its surroundings), and the o.d. is restraining the cylinder i.d. from fully expanding. Also, the piston face likely has less of a heat transfer path, thus gets hotter than adjacent cylinder wall.
In any case, having knowledge of the actual cylinder wall temperatures, and if possible the piston face temp's., will allow you to calculate the actual expansion of each component in service.
Lacking that knowledge, you will have to just cut and try. I.e., machine/grind a bit off the piston o.d. and see if it works.
A last wild idea - ever heard of piston rings?
RE: Problem in design of a piston and cylinder for elevated temperature
RE: Problem in design of a piston and cylinder for elevated temperature
Dear btrueblood,
The condition that you described is the case where the heat flows from the inner side to the outer face. But in our situation the heat is generated using band heaters that surround the outer dia of the cylinder, i.e. heat flow is from o.d to i.d.
In this case the cylinder expands more while the piston du to its lower temperature expands less. And this should encrease the tolerance. But in practice the problem remains.
RE: Problem in design of a piston and cylinder for elevated temperature
How long is the piston, and how well is it guided relative to the bore? Could it be cocking and binding?
What alloy are you melting (sounds like it might be Zamak? Aluminum?) and are you getting any oxide buildup that could be causing trouble?
RE: Problem in design of a piston and cylinder for elevated temperature
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Problem in design of a piston and cylinder for elevated temperature
I've done a lot of high temp conductivity, expansion, and fit stuff. This one smells a bit tricky because you can't get a good thermal measurement or calculation on the hot side because its molten metal.
It's probably a cut and try job as btrueblood says.
RE: Problem in design of a piston and cylinder for elevated temperature
I used to design aluminum extrusion dies, and in that industry they would heat the dies and backup pieces to temperature before every running aluminum through them. Just a thought.
James Spisich
Design Engineer, CSWP