How does a bottle jack work?

Hi kids:
I'm trying to design a foot pump jack for use on hospital gurneys to raise and lower the bed. It's set up just like a bottle jack and is
pumped up and then pressing a relief valve lets the bed back down.
The problem is, in looking at one, it doesn't appear to work like I thought. The pump is connected to the bottom of the cylinder and pumps oil under the piston. The intake of the pump is connected to a relief valve which is, in turn connected above the piston on the cylinder, sucking oil from the upper chamber. Simple, right?
Well, for some reason the upper port of the cylinder is connected to the *middle* of the cylinder (there is no port in the top cylinder head). So the piston actually passes over it on the way to the top, leaving the oil chamber open to both ports!
Are any of you familiar with this design?
Don
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On 23 Dec 2003 14:19:39 -0800, snipped-for-privacy@aol.com (eromlignod) wrote:

It seems you are *designing* this for sale. Am I correct?
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(eromlignod) wrote:

Yeah, what's your point? I've been a mechanical engineer for eighteen years designing industrial machinery. The hydraulic systems that I have been familiar with are larger, electrically-driven pumps with reservoirs, or air-over-oil systems. I haven't dealt with smaller, hand-operated systems like this before. All the other beds we make are electrically actuated. I was hoping that someone in the group was familiar with this type of setup and could save me having to dismantle the whole thing to figure out how it works.
Now do you know how they work, or are you just trying to bore everyone with your supercilious remarks?
Don
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On Tue, 23 Dec 2003 17:53:19 -0600, "Don A. Gilmore"

75 dollars per hour.

That shouldn't take a good engineer more than 20 minutes, would it? Perhaps you've been doing the same thing for too long. Thing is, you are doing this for profit and you are asking for help with the most basic part of the design. You are either lazy or incompetent. Which one is it?
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wrote:

By the way silly, you forgot to change your name back to the pseudonym.
Can you hear the trains from your place?
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I'm just posting from two different locations. The "pseudonym" is just my name backwards which is also my e-mail and what Google uses when I post from work. I don't really care which name is posted.
Why do you have to be so mean? I just asked a simple question to save some time. I answer questions on this NG all the time (look it up). That's what it's for.
I stand by my education and knowledge. I have numerous patents and have been interviewed by NPR and other radio stations all over the world. I have also had articles written about me in the New York Times and other newspapers and magazines in Europe, Asia and Africa.
Why don't you just let someone who knows the answer to my question have a shot and stop annoying everyone?
--
Don A. Gilmore
Mechanical Engineer
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On Wed, 24 Dec 2003 05:32:26 GMT, "Don A. Gilmore"

That's nice.

So? I've answered more questions in sci.* newsgroups than I've ever asked but if I were trying to design something for my own profit (as opposed to for some soulless company) I'd either do the work myself or compensate those who were helping me. I've seem to many people slime information out of people so that they could try their hand at becoming *rich*.
I guess the practice annoys me.

Ah gee, a celebrity. You weren't one of the Gilmore Girls, were you?

Because you amuse me. Dance some more Donny boy.
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Never dismantled one fully myself but here goes anyway. There are a couple of types of bottle jack. The design is different depending upon whether they are designed to work only upright or at all orientations. The latter are mainly used in engineering and industry for pushing as well as lifting. ASAIK, both are displacement rams. The piston is not sealed and only operates as a linear guide. It will have holes to allow oil to pass through it. All sealing is done by the neck seal. The pressure relief valve is connected from the high pressure side of the pump to dump to reservoir or the pump suction which is the same thing. The difference between the orientations is in the reservoir arrangement. The one I own has a free piston which is sucked in when the jack is extended so the reservoir is volume is reduced be the amount of oil pumped into the ram. Have you considered a remote pump jack in your design? It could solve your problems of design to use a bought-in item and cost less as a result. Hope this helps.
John
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Wow. Thanks John. You just saved me a lot of work.
The one I'm looking at has "Bimba"-type, sealed cylinders and I would have had to destroy them to figure that out. And you're right, making something like that ourselves wouldn't be very cost effective...I'd have to buy something off the shelf. The problem is that most off-the-shelf units are high-tonnage, which I don't need.
I had also thought of the remote pump/reservoir as a solution, but I wanted to fully understand this contraption first. Thanks again for saving me a lot of trouble/expense.
Don

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eromlignod,
If you don't know yet, you'll find out soon enough that this class of equipment attracts the attention of swarms of personal injury lawyers.
If you purchase a hydraulic lift unit for supporting humans, make absolutely certain that the manufacturer rates it for the purpose and get the certificate.
Cover your own part of the design, calculations, testing, procurement, and manufacture with bomb proof QA, and for Gawd's sake DON'T DISCUSS THE DESIGN IN A NEWS GROUP.
Just trying to help, Fred Klingener PE (ret)
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On Wed, 24 Dec 2003 15:42:12 GMT, "Fred Klingener"

Yes. I've been designing medical devices for the past 15 years (I've been an engineer for 20 years) and I personally know people who have been involved in the design of products that had *problems*. I've also been asked to falsify data that was being sent to a government agency. I quit the job instead. Grr. Dishonesty sucks.

Some manufacturers won't sell stuff to people who make medical devices. Some companies just charge more. They give a product two catalog numbers, one for medical and one for non-medical. I ran into this recently when getting reliability information for a power supply.

Especially when you are letting people know that you are trying to "save time". When you are designing a medical device you really ought to spend some time familiarizing yourself with stuff the FDA wants.
http://www.fda.gov/cdrh/index.html
Go there and search for beds. You really ought to look into the design control regulations unless you plan on just prototyping your idea and selling it to a company (like Stryker or the sister company to the Bates Casket Company - I forget the name - they are in Batesville, Indiana - Of course, if you have a really good idea and are a great engineer you could take it to Dean at DEKA - he might give you a job).
If you do this then you should patent it quickly and ... well, since you are already publicly disclosing crap you are probably in the gray area for being able to get European coverage already.
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absolutely
DESIGN
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I work for Medical Positioning (formerly American Echo). We manufacture hospital beds for echo-cardiograph, breast biopsy, fluoroscopic and other applications. You're absolutely right. It can get exasperating at times.
--
Don A. Gilmore
Mechanical Engineer
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On Wed, 24 Dec 2003 16:07:07 -0600, "Don A. Gilmore"

Ah, then you are doing this for a company, not for yourself?
A thousand pardons.
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The upper section of the cylinder above the piston is the oil reservoir. when piston reaches the hole it stops. ( no more oil to pump ).
very compact design, as reservoir gets larger as oil is returned from under the piston. and a built in limiter.
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Hi Jonathan:
Thanks for the response.
Well, that's the first thing I thought as well. But this thing is bizarre (at least if you can't see inside). The piston can go its entire stroke and you can even feel the "thud" of metal against metal when it stops at the top. It can travel a full 9.625 inches and the cylinder heads are only about 10" apart, which barely leaves enough room for a piston.
Like you said, if it were simply pumping from one side of the piston to the other, the piston would stop at the port and squeeze it off, so apparently there is no piston travelling past it. John Manders mentioned that the piston was merely a guide and that oil passes through it. This makes some sense, but then what raises the rod?
This is one of those things that once I see what's inside I'll say, "Aw, hell that's simple!" But, as I mentioned before, the cylinders are those permanently-crimped Bimba kind and I'd have to destroy them to see inside. This is driving me nuts!
Don
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under
Light dawns... :-)
Is this piston equipped with a large diameter rod ?, and is the centre port the *only* connection ?... if so and the piston is only a non sealing guide, then pressurising the entire cylinder will produce a force from the ram of pressure X rod area ]. The rod is retained by it's end guide / piston. -- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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port
guide,
Hi Jonathan:
No, we're still screwed. It has a normal-sized rod and the center port is not alone.
Actually the pump is built into the lower cylinder head. Ostensibly the oil is pumped directly into the cylinder from its suction line. The release valve is between the mid-cylinder port and the suction line.
Good thinking though!
Don
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OK, I finally figured it out. Here's a website explaining it if anyone is interested.
http://www.hyjacks.com/anjack.htm
Don

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Actually, I'm guessing that it does double duty as a strut, like those rear hatch things, but thicker and pumped, rather than leaky.
Putting the low pressure side in the middle of the cylinder would give you a pretty positive upward stop, while still keeping enough of the cylinder and piston [rod] engaged to strut effectively.
Mind you, I'm not an ME, my forte is electronics, but my favorite Xmas present as a kid was the No 6 1/2 Erector Set. This was about 1960, so I was 11-ish. My big bro (16-ish) did most of the work, but there was enough stuff for both of us to play with it simultaneously. Good Family Toy! :-)
Anyways, the above is my WAG based on nothing more than "intuition." :-)
Let me know what you think of it, OK? I'm more than happy to be enlightened. :-)
Thanks, Rich

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