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j king
11-29-2013, 09:34 PM
I am building a car scissor lift.I had to cut down the length of the used cylinders I picked up off a friend.I replaced the umbrella seals .The bores are 3 inches and the pump I got off my friend is a brown and sharp power unit with. 5 hp 3 ph motor and produces 1700lbs by my new installed guage.

Now the problem.I have this about 90 % done and wanted to give it a test with a car on it?Put my kids car on it and it would start to raise evenly then the right side would stop raising and left would keep on going.I have all new hoses and the feed line feeds a t-fitting and goes to the 2 cylinders so all is even as far as feeding the same.I tried switching lines and no change.I gave up and switched cylinders from side to side to see if something was binding and determined that the cylinder is malfunctioning.Acts same on other scissor so now the head scratcher .I took the cylinder apart and checked everything and all looks ok.When there is no load on the lifts they move fine all the way up and then collapse by just the lifts own weight so no binding .

Is there a way I can test the strength of this cylinder when I put it back together? I just don't see why it isn't pushing as hard as the other cylinder.

The way I proved to myself that it wasnt a mechanical binding was to separate the 2 scissors ."Torque tube tying them together"..
I put my lawn mower on one side and it wouldn't lift a 1000lb mower with the bad cylinder.I then switched the cylinder and the other side that would lift the car fine now wouldn't even lift the mower so no binding was proven.I am really stumped and have a huge oiled up floor from messing with everything.
Any ideas guys?

Ps.there is no oil bypassing the umbrella seals.These were double acting cylinders and The outer fitting is open and no oil is passing the seal.And one other thing.When it would raise part way up and I stopped the pump but kept the pressure on the cylinders it would hold the load and not drift down..makes no sense.

Cuttings
11-29-2013, 10:26 PM
I am probably covering what you have already done.
Please confirm these facts.
The cylinders are identical, so the surface area of the piston in each cylinder is exactly the same?
They are both mounted in the scissor lift so the leverage against them is identical?
The hydraulic pressure to both cylinders is identical?
If all this is correct the force exerted by each cylinder should be exactly the same thus the scissor lift should lift evenly.
If any one of these items is different from one side to the other the lifting forces will be different.
If none of this helps send us a picture. Maybe that will turn a light of for one of us.

Don Young
11-29-2013, 10:35 PM
It sounds to me like your cylinder must be binding under heavy load. Check that everything is perfectly aligned and that both the piston and rod bearings are not allowing the seals to be wedged because of side loading. Check the bearings for signs of excessive side loading.

j king
11-29-2013, 10:45 PM
What bearings are you referring to.when I shortened the cylinders I cut the Clevis end so rod and end bushing is factory machined.I checked the rod for straightness and it is good.The piston on rod run out is a couple thou.It all seems good.I agree that is seems to bind under a load but it is stumping me.

1-800miner
11-29-2013, 11:18 PM
Try turning the cylinder end for end and see what happens. Just a thought.
Measure the cylinder to see if it screwing up at exactly the same spot every time or if it varies.
And measure with different weight on the lift.
That may tell you where to look.
No dings in the barrel?

boslab
11-30-2013, 01:30 AM
Theres some crap or restriction in the exhaust or return side of the cylinder i reckon
Mark

Black Forest
11-30-2013, 01:59 AM
What mechanism does the scissor lift have to keep the cylinders synchronized? Is the valve and hose fittings and plumbing factory original? Synchronizing two hydraulic cylinders is not an easy task. A simple T fitting won't work in most cases. If the load differs much from one side to the other than the side with less weight will extend faster than the other causing the lift mechanism to cock. Remember that hydraulic cylinders don't create any force. The load creates the force. The pump only creates flow not force. There has to be a resistance to the flow of oil to create force.

There should be some sort of cable system maybe to keep the lift sides synchronized. So how is that done on your lift?

Jono
11-30-2013, 04:04 AM
I'm with BF. Mechanical synchronizers are more common, since hydraulic flow distibutors can slip a little. They usually consist of passive gear pumps similar to flow meters all mounted on a common shaft. The circuit is pump to manifold to distributor to cylinder. Should be fine for your job. If you can find two lowish displacement but identical pumps or motors you could make your own, since IME they are quite rare in scrap yards.

j king
11-30-2013, 08:45 AM
Well right now I am learning what I don't know.lol

The scissors are tied together at the pivot end by a heavy wall tube.It looked good in my mind but in reality the pivot end only moves very little in terms of rotation and it's full rotation is 45 deg so the two sides can't possibly stay exactly in time since the tube will twist in a few feet and installed a key and sleeve to join them.
I can deal with the cylinders not being in perfect rhythm by having two separate feed lines and control them separate if I have to.I just have to figure why the one doesn't have any power.I am still stumped as to the cause.

j king
11-30-2013, 08:49 AM
To make it clear as to what I am trying to accomplish here is a pic of someone else's lift.this is what I am trying to accomplish.http://img.photobucket.com/albums/v24/ikimjing/1383108156_zpsb4c96575.png (http://smg.photobucket.com/user/ikimjing/media/1383108156_zpsb4c96575.png.html)

vpt
11-30-2013, 09:04 AM
Both sides need a mechanical link to keep them even. Most lifts have a cable system tieing both sides together to keep them level with each other.

Jono
11-30-2013, 09:17 AM
An air lock would create what you describe. That's all I can suggest. Sounds like they're well enough tied together.

j king
11-30-2013, 09:33 AM
Jono , you have me thinking..I switched the cylinder in the almost fully extended position. I am betting it had air in it and when I switched sides it was still there and replicated the problem to the other side. Jeesh..sometimes you can't see the obvious !

j king
11-30-2013, 11:23 AM
Success !! Collapsed cylinder and it works now. I thought I had them bled when I started but missed that by a mile I guess.


http://img.photobucket.com/albums/v24/ikimjing/Mobile%20Uploads/1385831054_zps85ff0153.jpg (http://smg.photobucket.com/user/ikimjing/media/Mobile%20Uploads/1385831054_zps85ff0153.jpg.html)

http://img.photobucket.com/albums/v24/ikimjing/Mobile%20Uploads/1385830934_zpsdaf24136.jpg (http://smg.photobucket.com/user/ikimjing/media/Mobile%20Uploads/1385830934_zpsdaf24136.jpg.html)

Don Young
11-30-2013, 09:48 PM
That's a nice looking lift. I assume you have some kind of mechanical lock so it doesn't depend on the hydraulics to stay up.

I was referring to the piston and end gland wear rings or whatever kind of bearings (bushings) it has to take the side loading. Glad that wasn't the problem.

j king
11-30-2013, 11:26 PM
Have it covered Don.http://img.photobucket.com/albums/v24/ikimjing/Mobile%20Uploads/th_1383696985_zps2752b8e8.jpg (http://img.photobucket.com/albums/v24/ikimjing/Mobile%20Uploads/1383696985_zps2752b8e8.mp4)

vpt
12-01-2013, 07:49 AM
How are they linked to stay uniform to each other?

j king
12-01-2013, 08:44 AM
How are they linked to stay uniform to each other?

Look at first picture.they aren't connected but the bar across the two has a 1/2" slot on the ends of each tube and if you look close there is a 1/2 " key that fits in there .the shinny sleeve is then slid over the joint and key to hold key and provide support on the tube from spreading if twisted hard. Well that is my plan for how it should work.
When I tried it with the truck it would come even without the tubes tied together.

Have several hours left to do on them but wanted proof of function.hate to cut good cement up and it not work!cant wait til it's installed.

vpt
12-01-2013, 09:50 AM
Ah yes I see it now, at first I just thought that bar was to keep them together.

Don Young
12-01-2013, 09:53 PM
It is good that both sides seem to be equally balanced but never depend on hydraulics to do that. If the cylinders are in series they will work pretty well but will eventually get unbalanced due to small leakages in the cylinders. There are special cylinders with equalizing ports at the end of travel to help keep series cylinders equalized.

Parallel cylinders can completely extend one cylinder before the other moves, even with equal weight on them. Any slight change in the weight or friction can make it go the other way. If both cylinders are halfway out, a little extra weight on one will cause it to collapse and the other to extend. You therefore need a mechanical balance system strong enough to overcome any potential unbalance in the weight and friction.

j king
12-01-2013, 10:22 PM
Yes I want them tied together .I found this and I can't imagine it not having a tie.http://www.gregsmithequipment.com/Atlas-SLP-7K-Full-Rise-Scissor-Lift

wierdscience
12-02-2013, 12:10 AM
Yes I want them tied together .I found this and I can't imagine it not having a tie.http://www.gregsmithequipment.com/Atlas-SLP-7K-Full-Rise-Scissor-Lift

It's not really as critical as everyone thinks.If both cylinders are in good shape,with no dents or scores in the barrels and soft,compliant seals there is no reason why they would get out of time by any great degree.Locking prawls like those used in the lifts help matters as do the pilot operated load locks many of those lift pumps have built into their valve bodies.

http://www.metromachine.com/Hydraulics/Lock%20Valves.pdf

Black Forest
12-02-2013, 03:53 AM
It's not really as critical as everyone thinks.If both cylinders are in good shape,with no dents or scores in the barrels and soft,compliant seals there is no reason why they would get out of time by any great degree.Locking prawls like those used in the lifts help matters as do the pilot operated load locks many of those lift pumps have built into their valve bodies.

http://www.metromachine.com/Hydraulics/Lock%20Valves.pdf

It has been my experience that synchronizing two hydraulic cylinders is indeed a problem. If the two sides of the lift were not tied together with a robust system if the weight of one side was significantly different the light side would raise first or faster. It costs a lot of money to accurately synchronize two hydrulic cylinders. Involving sero type valves, etc., etc..

vpt
12-02-2013, 08:54 AM
With a vehicle lift you NEED the sides tied together. The vehicle off to one side by 1" will load that far side by 100 pound or more than the other side and cause a problem.

Pretty much every vehicle out there is not equally balanced from one side to the other.

I can tell when lifting vehicles with my lift how much strain is on the cable tie system because the vehicle is off to one side a little bit or the like.

Plus normally you are working on the vehicle when it is on the lift. Take the tire off one side and then what happens? Go further and take a whole axle assembly apart on one side. Pull a motor out of a FWD car, pull the trans, hell even doing a timing belt and stepping up on one of the lifts will cause that side to go down.

Bicycle Man
12-02-2013, 09:23 AM
This is an easy fix. You need to split the oil from the pump into 2 equal streams of oil. If you have a 6 gpm pump you need to split it into 2 streams of oil 3 gpm each.

Get 2 hydraulic pumps or 2 hydraulic motors either one will work. Make sure they are both the same. It makes no different if you have 4 GPM or 6 or 10 or 15 GPM just as long as both are identical. Connect the shaft of each hydraulic pump together so when 1 turns the other one has to turn too. This is called a regulator.

Pipe the oil from your hydraulic pump to a T fitting then from the T to the regulator. Oil from the regulator goes to each cylinder. The regulator makes sure both cylinders get the same amount of oil.

Ok it will work fine like this as long as there are no internal oil leaks. If the seals start to leak on one cylinder then it screws up the whole system one cylinder will go faster than the other. You need a dump valve on both cylinders. Dump valve is a very tiny hydraulic valve that drains a tiny amount of oil out of the cylinder that is trying to go faster than the other cylinder. It drains a little oil back to the tank to slow the fast cylinder down so both cylinders go the same speed.

You need a cross over system from cylinder 1 to 2 and from 2 to 1. Cross over system is just a mechanical linkage between lift 1 and lift 2. If lift 1 tries to get ahead of lift 2 the linkage between 2 and 1 opens the valve on 1 to make it slow down to the same speed as 2. If 2 tries to go faster than 1 linkage opens the valve on 2 and slows it down. Simple little trick. The cross over only needs to be able to drain off 1 tablespoon of oil per minute to make both cylinders run the same speed.

I use to work for a hydraulic company. You can buy a regulator and a dump valve. You can use a duel pump as a regulator. A factory made regulator has no external shaft. If you try to buy one they will want part numbers, salesman won't have a clue what you need without numbers. You can build your own just by knowing how it works.
















I am building a car scissor lift.I had to cut down the length of the used cylinders I picked up off a friend.I replaced the umbrella seals .The bores are 3 inches and the pump I got off my friend is a brown and sharp power unit with. 5 hp 3 ph motor and produces 1700lbs by my new installed guage.

Now the problem.I have this about 90 % done and wanted to give it a test with a car on it?Put my kids car on it and it would start to raise evenly then the right side would stop raising and left would keep on going.I have all new hoses and the feed line feeds a t-fitting and goes to the 2 cylinders so all is even as far as feeding the same.I tried switching lines and no change.I gave up and switched cylinders from side to side to see if something was binding and determined that the cylinder is malfunctioning.Acts same on other scissor so now the head scratcher .I took the cylinder apart and checked everything and all looks ok.When there is no load on the lifts they move fine all the way up and then collapse by just the lifts own weight so no binding .

Is there a way I can test the strength of this cylinder when I put it back together? I just don't see why it isn't pushing as hard as the other cylinder.

The way I proved to myself that it wasnt a mechanical binding was to separate the 2 scissors ."Torque tube tying them together"..
I put my lawn mower on one side and it wouldn't lift a 1000lb mower with the bad cylinder.I then switched the cylinder and the other side that would lift the car fine now wouldn't even lift the mower so no binding was proven.I am really stumped and have a huge oiled up floor from messing with everything.
Any ideas guys?

Ps.there is no oil bypassing the umbrella seals.These were double acting cylinders and The outer fitting is open and no oil is passing the seal.And one other thing.When it would raise part way up and I stopped the pump but kept the pressure on the cylinders it would hold the load and not drift down..makes no sense.

wierdscience
12-02-2013, 09:45 AM
It has been my experience that synchronizing two hydraulic cylinders is indeed a problem. If the two sides of the lift were not tied together with a robust system if the weight of one side was significantly different the light side would raise first or faster. It costs a lot of money to accurately synchronize two hydrulic cylinders. Involving sero type valves, etc., etc..

Only if the load is highly unbalanced or balanced,but very lightly loaded.The Scissor will work fine since it's essintially loaded even when it's not lifitng anything due to mechanical disadvantage.

If the seals were passing enough fluid to get that scissor system out of sync,having it tied together would be a problem.Reason being the operator would have no way of knowing that one cylinder was carrying the whole load until it failed,suddenly.

Those pilot lock valves are cheap in the small sizes we are dealing with here(<$100)and are very effective at maintaining a pressure balance between cylinders,even when the cylinders are not the same exact bore size.

Understand though I'm not talking about maintaining a tight tolerance.If one side of the car is a 1/2 higher than the other it's no problem,different story if they need to be with in .001".

Black Forest
12-02-2013, 11:58 AM
This is the simplest way to synchronize two cylinders that I have found. If the cylinders are tight and not leaking past the seals it works fine.
http://i853.photobucket.com/albums/ab91/burnandreturn/sychronizinghydrauliccylinders_zps97d431f7.png (http://s853.photobucket.com/user/burnandreturn/media/sychronizinghydrauliccylinders_zps97d431f7.png.htm l)

wierdscience
12-02-2013, 01:13 PM
Yup that one works,here is one using the lock valves,same principle for single acting cylinders,just half as many valves.
http://img.photobucket.com/albums/0903/wierdscience/PILOTCHECK_zps2838c829.jpg (http://smg.photobucket.com/user/wierdscience/media/PILOTCHECK_zps2838c829.jpg.html)

One could also simply add two ball valves one either side of the tee and manually re-phase them if they drift out of time.The one saving grace in the scissor system is that both cylinders land on a hard stop which automatically re-phases them everytime they park.

Nice job building the lift J King!

j king
12-02-2013, 02:24 PM
I can't read hydraulic schematics.

Thanks Weird.