Hydraulic Tidbits

MossRoad · Oct 23, 2002

I was talking to my brother-in-law last night about my grapple project and other fun stuff. I mentioned the hydraulic cylinder that I'm planning to use. He told me this little tidbit on hydraulic cylinders that I never knew. Is it true? Seems to hold water...

A typical hydraulic cylinder is stronger on the extend stroke than the retract stroke. Why? Lets say you have a 2" cylinder with a 1" rod. When you extend the cylinder, the hydraulic fluid has a full 2" disk to push on. When you retract, the fluid is pushing on the other side of the disk. The center of the 2" disk is occupied by the end of the 1" shaft that is attached to it. There is less surface area for the fluid to push on, so it is not as "strong".

I never thought of that. Hmmm.

davesisk · Oct 23, 2002

MR:

That sounds correct to me. The surface area of the disk is 2*pie*R^2 (where R is the radius, or half of the diameter). So the 2" disk has 3.14 sq inches on the push side, but only 2.355 sq inches on the pull side (3.14 outer circle minus 0.785 for unavailable 1" diameter of the inner circle), or only 75% of the same surface area (and thus force, I'd imagine).

Since the force is some function of pressure X surface area, this really seems to make sense. I'm going to have to go look this up on <A target="_blank" HREF=http://www.howstuffworks.com>http://www.howstuffworks.com</A> now that you got me wondering about the specifics.

Also, since the volume of hydraulic fluid that the side with the rod can hold is smaller than what the side without the rod can hold, this might be a factor also.

Can anyone else validate?

Dave

Charlie_Iliff · Oct 23, 2002

<font color=red>Can anyone else validate?</font color=red>
Dave: I think you have a typo: an extra 2 in the formula, but your calculations are right. It's pi*r^2, and that's what you actually used. I'm not sure of friction losses, etc, but certainly a double acting cylinder has more force, and is significantly slower in extension than in retraction, due to the differing piston area and chamber volume.
But, of course, I'm not qualified to "validate". That probably requires some tests and a video on MR's web site./w3tcompact/icons/smile.gif

MossRoad · Oct 23, 2002

Dave,

I forgot all about pi times the radius squared for area of a circle. Thanks for the calculation.

Charlie,
That makes sense. Extension is more powerful, but takes longer, while retraction is faster, but less powerful.

MossRoad · Oct 23, 2002

Ever wonder why our loaders seem more powerful on the curl up than the dump down? I always thought the linkage on payloaders was for torque amplification. I never could figure out why they had all that linkage instead of just putting the hydraulic cylinder directly to the bucket. Well, if they did, it would have more power down than up. Look at the attachment.