Say that I have a 20T jack. If it has a 4" dia slave cylinder, that
means a (max) pressure of 20 x 2000 / (PI x 2^2) = 3182 PSI. If the
master cylinder is 1/2" dia, then the (max) force on it would be 3182 x
(PI x .25^2) = 625 lbs. If the operating lever has a 12 to 1 advantage,
it would require 625 / 12 = 52 lbs on the lever to generate the 20T lift.

All well & good, but what happens if I put more than 52 lbs on the
lever? I.e., what's the 1st thing to go? A piston seal, I suppose -
but is that right? Anybody ever done it? I.e., tried to squeeze that
little bit extra lift & went too far?

And if it does leak by a seal, is it ruined?

Or is there a pressure relief valve in there somewhere that protects the
seals? Probably not on a HF jack.

Inquiring minds ...,
Bob

On Dec 24, 6:31*pm, Bob Engelhardt wrote:
...
Or is there a pressure relief valve in there somewhere that protects the
seals? *Probably not on a HF jack.
Bob

I've tripped the pressure relief valve on an 8 ton long travel jack of
the type HF sells.

jsw

Bob Engelhardt wrote:

Say that I have a 20T jack. If it has a 4" dia slave cylinder, that
means a (max) pressure of 20 x 2000 / (PI x 2^2) = 3182 PSI. If the
master cylinder is 1/2" dia, then the (max) force on it would be 3182 x
(PI x .25^2) = 625 lbs. If the operating lever has a 12 to 1 advantage,
it would require 625 / 12 = 52 lbs on the lever to generate the 20T lift.

All well & good, but what happens if I put more than 52 lbs on the
lever? I.e., what's the 1st thing to go? A piston seal, I suppose -
but is that right? Anybody ever done it? I.e., tried to squeeze that
little bit extra lift & went too far?

And if it does leak by a seal, is it ruined?

Or is there a pressure relief valve in there somewhere that protects the
seals? Probably not on a HF jack.

Inquiring minds ...,
Bob

Relief valve. Nothing more than a steel ball and a spring under a plug,
even the cheap jacks will have them.

Pete C. wrote:

(...)

Relief valve. Nothing more than a steel ball and a spring under a plug,
even the cheap jacks will have them.


Yup. My cheap 2T toe jack goes into bypass mode easily, when overloaded.
Once lowered and repositioned it works just fine once more.

It is 'just the ticket' for disassembling cinder block planters.

--Winston

--

Congratulations Robert Piccinini and Steven A. Burd, WalMart Publicists of the Year!

Thanks for the replies. I'm glad that I asked - now I can hang on that
20T'er that's on my press & not worry about breaking it.

Bob

Bob Engelhardt wrote:

Or is there a pressure relief valve in there somewhere that protects the
seals? Probably not on a HF jack.

I have some chinese jacks. THey have the relief valve.

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

Bob Engelhardt fired this volley in
:

Anybody ever done it? I.e., tried to squeeze that
little bit extra lift & went too far?

And if it does leak by a seal, is it ruined?


Bob, by all rights, it must have a pressure relief valve. Almost always,
over-pressuring an o-ring seal will blow it out, resulting in a sudden
loss of pressure, and a dropping of the load.

My cheap Pep-Boys jacks have relief valves.

LLoyd