linear actuator

[marmon]

heres the problem imagine a 75 degree angle flat on the bottom with one leg on a 75 degree rise a linear actuator sits inbetween this angle to make it smaller, down to 15 degrees. one end on the horizontal leg and the other directly on the 75 degree leg. heres the problem the actuator starts off past perpendicular to the bottom leg and when it contracts as it goes past the perpendicular point on its way to colapsing it binds and is breaking. is this because of loose bushings or is it just the nature of a link passing the perpindecular point???

 

[gwolf]

A picture is worth a thousand words.

 

[ccw]

Hi Marmon,
Even the best hydraulic actuators and pivots are subject to column buckling failure. Most linear actuators are rated to withstand full piston area x max rated press.= force at full extention + some safety margin, as a pinned-pinned column. But, it is easy enough, given a range of geometries, to exceed that column buckling critical force. Usually though, the column buckling induced failure occurs when the actuator is near full extension, not near full retract.

The other culprit could be an extraneous mechanical interference encountered with the actuator body, the rod end knuckle, the closed end knuckle; or one of the members moving out of plane as the actuator retracts. Look for skinned paint or bright metal marks around the actuator and adjacent structure as clues. And, make sure the two legs remain in plane.

You should visit your friendly hydraulics shop and get a couple of tees and cheap pressure gages and put them in the actuator A and B ports. Map the two pressures throughout the stroke...this may isolate and point to your problem.

 

[MikeHalloran]

It sounds like the mechanism forms a toggle that works against the actuator. You can do a simplified analysis in Excel. Apply an arbitrary load, or the weight of the moving assembly, as appropriate, and calculate the force required for the actuator to hold it in equilibrium over a selection of positions within its range of motion.

You will find a range near the 'binding' position where the actuator force requirement becomes very, very large. For an exact analysis, you'd have to account for the stiffness of associated structures in that range, but in this case you don't need to; you've already demonstrated a catastrophic failure mode.

It's probably time to reconsider the geometry of the linkage.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[zekeman]

If you want answers, why don't you explain what you have more clearly including the angles the mechanism makes in the overcenter position and the bearing configuration
I'm sure that Mike is right on target in that for very small angles, the mechanism will go to to a locked position due to finite friction. The toggle condition would be a zero angle which would be more obvious.
Suppose the overcenter angle between the almost vertical link and the actuator is theta, then I calculated that for very small angles, in order to avoid locking, the coefficient of bearing friction must be less than
L*theta/2r
where
L= length of near vertical link
theta = angle inradians between link and actuator
r = radius of bearing