Tapped holes and fatigue

Question

Tapped holes and fatigue

KDB_17 (Mechanical)

(OP)

18 Jan 18 16:37

I am currently designing a system that requires an actuator to be fastened to a horizontal baseplate via a clevis plate. During operation, the actuator will transmit cyclic loading to the baseplate (normal to baseplate) with a high force. After a significant number of cycles, the actuator and clevis plate will be replaced but the intention is to keep using the baseplate.

My question is: can I use tapped holes in the baseplate, i.e. can tapped holes be re-used for an application where fatigue failure is a risk? (the baseplate itself is designed such that it is below the endurance limit). Because of space issues I can't get a nut on the underside of the baseplate.

Thanks in advance, KDB

jgKRI (Mechanical) · Answer 1 · 2018-01-18 16:42:10

Ask yourself this: is the thread actually seeing a reversing load?

KDB_17 (Mechanical) · Answer 2 · 2018-01-18 16:55:13

Not directly axially - however the top of the plate is alternating between tension & compression - I don't know if this would cause an issue in the surface with the tapped holes?

Also, I don't know what the cumulative effects on the thread are when re-using the hole for this kind of application

Tmoose (Mechanical) · Answer 3 · 2018-01-18 17:24:17

Based on what I'm imagining (**) your arrangement looks like, I'd say if the bolt clamping force is "high enough", the friction between the clevis plate and baseplate will easily resist all anticipated and unanticipated transverse forces, and the bolts and threaded holes will be subjected to negligible alternating forces. The bolts also will never loosen in service, so "locking devices" should be used only for bolt retention, just in case, if at all.

(**) http://www.newtonsapple.org.uk/wp-content/uploads/...

desertfox (Mechanical) · Answer 4 · 2018-01-19 06:46:58

Hi KDB 17

No reason why you can't use tapped holes rather then nuts, however depending on the material the tapped holes are in you would need to check the shear stress for that material and compare it with that of the nut mterial.
I the preload is high enough on the bolt then the cyclic stress in the fixings should be quite small, it is interesting to note that when you tighten on a female thread whether it's a nut or tapped hole the stress is actually compressive unlike the bolt which is in tension.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

KDB_17 (Mechanical) · Answer 5 · 2018-01-19 08:29:50

Thanks for your replies Tmoose & desertfox

racookpe1978 (Nuclear) · Answer 6 · 2018-01-19 16:59:45

Can you add two (or four) pins (dowels or press-in rolled pins) to take the changing (sideways) shear forces? That way the clevis assembly is held "down" (clamped to the baseplate) by the tapped holes (always compression forces) but the pins take the changing sideways forces.

Tmoose (Mechanical) · Answer 7 · 2018-01-19 18:14:51

The combustion and inertial forces (hundreds or thousands of pounds) applied to a typical non-opposed auto engine main bearing cap are mighty significant and highly alternating, primarily in one plane.

Two bolts per cap proved sufficient for a bunch of years.
http://blog.ecklerscorvette.com/wp-content/uploads...

The cap sits snugly in a shallow register in the engine block, but that is to maintain accuracy of the machined hole ( 0.0005" or less ) thruout repeated disassemblies, not to resist the considerable forces in operation.

Bolt clamping forces, for decades controlled by installation tOrQuE, applied to decently machined faying surfaces created friction and was dead reliable.

BrianE22 (Specifier/Regulator) · Answer 8 · 2018-01-19 18:19:34

I'd be more worried about wear on the flanks of the threads. Use a lubricant if you aren't already.

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