Hydraulic Scissor Lift 7

[AC-Power]

Good Day All,

I am not a mechanical engineer, rather, I am electrical. Having said that, I am hoping there is someone in here that might have fluid mechanics experience.

I just bought a scissor lift from an auction. When I attempt to drive forwards or reverse, all I hear is the 24VDC hydraulic motor start, no movement. Too, when I attempt to raise the platform, same thing.

Is there anybody in this forum who might have a suggestion on what part I should investigate/troubleshoot in an effort to remediate this problem?

Regards to All

 

[AC-Power]

The hydraulic pump is a positive displacement pump.....should it freely spin manually with only muscular force? The input and output oil line hoses are still connected, I haven't touched them. I don't feel it building pressure and producing resistance when rotating it by hand. Is this right?

 

[3DDave]

It's probably time to find a hydraulics repair shop. I would not expect a gear pump to be easy to turn in either direct by hand, though it's never come up for me to do so, but I also don't know of a calibration I could give to measure how "easy" it should be. If the pump is damaged internally that would mean fragments could be carried into the rest of the system, potentially damaging valves.

 

[LittleInch]

AC Power

Lets go back to basics here.

At $4K you probably aren't going to send this into a proper repair shop unless you've got another $4-5K spare??

The pump should be producing flow and if controlled correctly a lot of pressure. So the easist to try first is to disconnect the outlet hose of the pump and turn it on directing the fluid into a bucket or back into the tank. Do you have flow Y/N

If you have flow then do you have pressure? Bit harder as there are no pressure guages, Things to look at on the pump - Does it have an internal pressure relief valve built into the pump? If so check it out to see if it's broken / jammed open.

The next port of call is the pressure compensated proportional valve. This as shown on the hydraulic diagram ( you called it the electrical diagram) has two functions.

The first item regulates the pressure downstream to be a fixed pressure at the same flow. This means your lift should go up and down at the same speed regardless of load. This one appears to have the electrical signal from the enable switch and hence only allows hydraulic oil through when the switch is depressed.

The second part of it appears to maintain a fixed pressure on the output of the pump, so that when the pump is running, but there is no fluid moving into the machine, it sends the flow back into the reservoir. If this part of the valve is stuck open then all your fluid will simply go round in a short circuit.

Also even if your valve is working, is the R1 system relief valve working correctly? Again a possible short circuit for your fluid.

I would find out how much these two items are and replace them once you know the pump has flow. Then you can be reasonably sure that your hydraulic system has pressure available and can send flow to the pistons or the wheel motors.

Then it's a matter of trying to find out what else is wrong...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AC-Power]

Thanks So Much 3DDave and LittleInch for your advice. I really do take it seriously, and I apply your suggestions each day (or the next day, time permitting) that you recommend. The two of you are far smarter than I am with this. Naturally, I do good with electrical, but my experience in fluid power is quit limited. As this lift is owned by my company now, I would like to "tackle" this problem myself so as to learn how to fix this thing in the future. I own a shop with milling machines, metal lathe, metal bandsaws, 3 welders, knee mill, hydraulic press, shear, brake, water cutter, etc.....there should be no reason I can't do this.....at least that's what my wife says!

I'll try your suggestions this afternoon (Sunday May 16).

Thanks Again To All.

 

[3DDave]

Considering the Skyjack manual is cautioning about not letting lint into the hydraulic fluid (probably due to small clearances needed for the proportional valves) it's not the best candidate for a first timer. OTOH - it's not huge expense compared to typical hydraulics.

At its heart, it's just plumbing that can make a big mess. The top concerns for hydraulics is that the actuators can kill you, either because the system is operating correctly or because it is not. And high-pressure leaks can also kill you or do grievous harm - a risk that is most often the case when there are external loads, operating pumps, or hydraulic accumulators.

Typical example of bad is unscrewing a fitting from a hydraulic cylinder to see why it won't retract when there is a load on it. Or seeing a wet spot or drip on a high-pressure hose and wiping across it with a finger only to find it's a 3000 psi jet that's nearly invisible that injects fluid into the hand. Or bypassing a safety switch while reaching into equipment.

OTOH, this unit in the retract position and motor off has no obvious sources of stored pressure or releasable energy - so it's just messy, hence the suggestion for the bucket.

Just make sure the reservoir doesn't get empty if the pump is operating and don't leave the platform elevated when undoing any fittings. And get in the habit of never checking any drips by hand without knowing the pressure is relieved.

See:

https://cse.umn.edu/me/intranet/operations/safety/training/hydraulics

https://www.hydraulicspneumatics.co...40/fluid-power-safety-in-the-workplace-part-5

 

[3DDave]

This might be a worthwhile purchase:

https://www.amazon.com/INTRODUCTION...draulic&qid=1557841995&s=gateway&sr=8-1-spell

 

[AC-Power]

My replies are in RED:

Ok, I did perform the first suggestion of LittleInch's most previous comment:

LittleInch = "The pump should be producing flow (It does)and if controlled correctly (It is. Pump Inlet from reservoir, pump output to pressure compensated proportional valve, motor turning correct direction, all good) a lot of pressure. So the easist to try first is to disconnect the outlet hose of the pump and turn it on directing the fluid into a bucket or back into the tank. Do you have flow Y/N" I bench tested the pump (& motor) by manually dumping hydraulic oil into the input hose (gravity assist), it sprayed well into a bucket, hard. Replaced the pump (& motor) into the lift, applied oil again manually (with a funnel, gravity feed into pump) meanwhile I GOT A HELPER TO MAINTAIN A HYDRAULIC COMMAND (in this case, turn left). So I was manually presenting oil to the inlet of the pump AND requesting a hydraulic command simultaneously, the oil sucked into the pump VERY quickly and returned to the reservoir instantly, WITH NO MOVEMENT OF THE LEFT TURN cylinder. I am wondering if I should look at replacing the system relief valve? Could it be stuck open?

BTW, the left turn solenoid is receiving 24vdc upon its request for opening, and I hear it clicking.

Thanks For Your Help to All of You!

 

[3DDave]

Time to find out if the fluid is not leaving the tank into the pump. Check to see that the dip-tube is still attached and not plugged. I'd first try with some tubing to attach to the upper end of the dip tube and see if siphoning into a bucket works. If it does then you should re-secure the pump inlet hose to it. If not, remove the screws holding the tank lid in place and see if the dip tube is still there and not plugged.

It is certainly possible for a previous worker to have lost a rag or paper towel in the tank. (Saw an entire 350HP diesel severely damaged because a rag was used to stop a drip when a cooling hose was removed. It blocked the cooling passages in the head after the hose was reinstalled.)

Also - that's supposed to be a 4 HP electric motor. The pump can make that power available anywhere the fluid can go, so remain cautious. As you noted, there was some forceful output.

 

[LittleInch]

Well that's a start.

So now, assuming the pump is back in it's correct location, try and work out if the flow is coming from the relief valve or not. This might be more difficult, but at least you should then know if the pressure compensating valve is working or if that is stuck open and just returning all the fluid back into the tank.

I think a good investment would be a pressure guage attached to a male hydraulic connection so that you can check if any particular valve is seeing hydraulic pressure at the end of the connecting hose. Don't know what your system is intended for but suspect you're looking at a few thousand psi.

You just need to follow the path of the oil and see if ther eis pressure and /or flow from each part.

My current suspicions are your proportional valve is jammed open or the relief valve is jammed open, but might not be either of those things.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AC-Power]

Ok, Thanks Again LittleInch. I'll go get a pressure valve today, and the necessary fittings. I'll let you know.

Again, I take your suggestions very seriously....you are FAR more knowledgeable on this than myself as an electrician.

Best Regards

 

[AC-Power]

Ok, I finally got a pressure gauge. Checked the output at the proportional (prop) valve, nothing. Upon any command for hydraulic pressure, the prop valve routed the oil back to the tank. I checked for DC power at the prop valve, nothing! I thought I checked that before, but it appears I must have either misread the meter, or I checked the wrong solenoid in thinking it was the prop valve. I traced the valve's DC power back to a dysfunctional automotive relay (terminals 30, 85, 86, 87 and 87A). Changed the relay, all UP and running.

I would not have been able to move along with this repair without the help of 3DDave and especially LittleInch. Thanks So Much for your help. If you have any need for assistance with electrical, feel free to ask me in this forum. I am mostly involved with civil electrical work, such as traffic lights, induction loops, LPR, toll controllers, classifiers, street/highway lighting, optical preemption, GPS preemption, etc. I specialize in traffic control systems, even things such as parking garage ticket dispensers, gates, pay stations, LPR, etc.

Again, Thanks to both of you.

 

[LittleInch]

AC-Power

No problem, glad we could help and it's great you came back and updated us, You wouldn't believe the number of posts that just stop without any conclusion or response.

Fault finding by remote control is not easy as you've found.

You might find you can add value in this forum

https://www.eng-tips.com/threadminder.cfm?pid=163

And now you've got yourself a hydraulic lift, try the fluid power forum....

Send a picture or two with it working at max height!

LI





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AC-Power]

Ok, I will do that.

Once it quits raining here in Calgary, AB, I'll take a pic or two. It started yesterday, and is supposed to keep going until Wed/Thur.

Too, I will check out the "threadminder" link you've suggested in this ENG-TIPS forum.

 

[LittleInch]

Ha - It's raining here as well as it has done all of May....

The other forum I linked to (Traffic engineering) doesn't look like it gets much traffic #(excuse the pun), but other forums may interest you as well as some of the civil ones get into bridges and other traffic related stuff.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AC-Power]

Ok, as promised here is the picture of the recently repaired lift raised to 26' (platform height) above grade. I repaired this in my home's driveway, so the o/h lines feeding my house are in the picture.

Again, thanks for your help! I TOTALLY benefited from your help!

I have another post...a math question, so I might try to close this thread (if I can) as I am understanding I cannot post (be the OP) to two (2) threads. I would need to see if there is a "math" savvy engineer in some room/thread in here. I could probably figure this out myself (after about 8-10 hours of searching), but I suspect a "math" type engineer could do it in a manner of minutes. Failing that, I will call our P Eng to figure it out for me.

 

[3DDave]

The idea is not to post the same question to multiple places. Asking different questions is fine.

 

[itsmoked]

Just found this thread. Bravo AC-Power! I've had to troubleshoot lifts several times. They are often a PITA to figure out.

I'm also jealous you found a lift as I spent a couple of weeks trying to hunt one down. Even used junk is typically $4~5k. I finally found a very nice new one for sale for about $2k only to discover shipping was $6k! I finally gave up.

You have a pretty cool job nitch you've made too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

Ac power.

Thanks for that.

You can have as many threads open as you want.

Threads close officially after 6 months when you can no longer post replies but in reality most just naturally stop and gradually fall off the first page.

The key is to only deal with one question in each thread and only post the question in one forum. Hence you need to choose your forum with care.

So figure out which branch of engineering your maths related to and post it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AC-Power]

Thanks itsmoked. The problem with the lift actually was nothing hydraulic....it was just a bad ZETTER relay (automotive type, black cube, 30amp, one set of form C contacts). I bought the lift from Edmonton, approx a 3hr drive away from Calgary, but I have a 1-ton dually truck, so I got a local 3rd party crane service to lower the lift into the truck bed. The lift weighs 4700lbs, but the truck can carry 7000lbs payload. Anyways, I hope you find a lift so your business, and too, I hope your business goes well after this COVID thing is over.

 

[itsmoked]

Thanks AC! I'll keep my eyes peeled. My business actually exploded about March 2020 and I'm so busy I'm treading water. Not even sure why it got busier.. LOL

Keith Cress
kcress -

http://www.flaminsystems.com