huangyhg

elevated temperature tolerances!
hi friends,
i have question about the effect of temperature change on the standard 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 using this tolerance, when these parts bring into those high temperatures they stick each other, preventing the piston from slinding.
i wonder if the tolerances are not correctly chosen?
i am looking foreward to your opinions.
vahid.

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you're right. the tolerances are not correctly chosen.
i'd suggest calculating the (approximate) expansion of the nominal size piston & cyliner at operating temperature and then using that to drive the tolerance, perhaps iterate a couple of times if need be.
don't get obsessed with using 'standard fits' if your situation isn't standard.
you could of course specify that the dimensions apply at 600c (by default as i recall they apply at 20c) however, that is either shirking your duty of doing the calculations and/or expecting some tricky inspection! the expansion due to heat is a design issue and so you should probably look to solve it. i believe similar questions about thermal expansion may have been posted before, perhaps take a look on here.
kenat,
i would scale the part in a cad system by (1-(alpha x delta t)) where alpha is the cte. the desired tolerances should be the same as needed but the nominal size would be scaled using this technique.
search online for % of temp change.
dear friends,
thank you for your useful tips. i have already 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.
maybe, the parts distort or bend during heating due to internal stresses release. evey bar has internal stresses balance due to the manufacturing process, etc. the hole drill and any other manufacturing process change the internal stress balance. when it is heated due to the molten fluid the internal stresses balance change again. this can change the hole diameter or even probably the straightness of the cylinder or the piston rod. the tighter the tolerances the risk for interference grows.
sounds like this is more of a design problem than a tolerance specification problem.
while you may get the answer you need here you'll find a larger audience of mech engineers somewhere like

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