Hydraulic load cel

[JamesA]

I designed and now testing a mechanical (piston type) hydraulic load cel, but once a load is applied, I experience a pressure loss of about 25% before pressure levels off and will hold steady for a long time (one tested 24 hours). When filling, I tried to top it off with hyd fluid, but have a hunch air is entrapped in the cylinder. Any suggestions to fill and getting 100% air out? I have search for a pressure guage that I could bleed off the air, but no luck. I have heard of vacuum filled piston, but no idea how this is done.

[PinkertonD]

have a hunch air is entrapped in the cylinder

Hi James, I suspect it could be described as more than a hunch.

You will need to add in a bleeding device. A simple one would be a drilled hole through, then tapped half way down, a ball bearing that drops in to it, and a bolt to clamp off after no more air appears. It is terribly optimistic to think you can fill any hydraulic system without needing a bleeding system.

Ideally there should also be a method of presurization of the fluid entering to ensure all air is driven out.

[RWOLFEJR]

Hi James and welcome to the forum...!
Just curious...
I'm wondering if there isn't something else you should be looking at? (Or maybe I'm misunderstanding what you've got going on there... Which is always a strong possibility.) Pressure is pressure is pressure... Air will allow movement as it compresses but the pressure will be the same? The air will act like a spring but the load will still be the same. For work being produced from hydraulics you're typically after a "nearly" rigid or solid motion and then yes... you need to purge any air out to stiffen things up, but the presence of air will not reduce the pressure. A 100 lb. weight on an air cylinder vs. 100 lb. weight on a hydraulic cylinder is same / same. Only difference is the piston of an air cylinder of equal dimensions to the hydraulic cylinder will travel further before it smashes the air under the load.

Any chance of an error in calculations for pressure vs. square inches of surface that pressure has been delivered on? Or like I said... maybe I'm way off?

[jboggs]

RW - Good catch!

JamesA - Hydraulic load cell? How is this not just an electronic pressure gauge? And if it is, why not just buy one?