Asking For Ideas About Hydraulic Lift Table Troubles

[PGB1]

Hi All!
I apologize if this is in the wrong forum section. I considered the accessories section, but that seemed more for actual machine accessories.

Several years ago I was given a hydraulically lifted scissor table which is an older version of Harbor Freight #69148. When last I used it, it worked fine. But it was not used for a couple of months & now won't lift. There were no oil drips under it and the reservoir was full when I removed the fill cap.

I am guessing that many people here have hydraulic equipment in their shops and may have encountered the problems my table is presenting. Hopefully you all will see where I'm going wrong with my attempted repairs.

I tried purging air with every method I could find.
In the past, the most success I've had is to: Fill the cylinder, put the fill plug in, hold the release (lowering) lever while pumping slowly about 20 times. Then I'd remove the fill plug while holding the release lever to let air escape. Then I'd repeat as needed. No luck this time.

I removed the cylinder and took it to the bench for testing. I drained the fluid to look for chips or other debris & to get a new start with fresh hydraulic oil (ISO 32). It is set up at the angle it is while in the table. (Photo Attached)

Here are my amateur's diagnostic steps:
On The Bench:
I repeated the air purge procedure. No air was expelled.
While pumping, some light pressure had to be on the ram, pushing it back into the cylinder. If no pressure was applied, the ram would not move. Very light palm pressure worked and the ram pumped up and down quite well. It would go to it's full travel distance.

When pumped up, the ram held fast & fully extended until the release lever was pressed. Then, I could push the ram back into the cylinder. The last 1/2 inch was quite hard to push.

Back In The Table:
The cylinder would not lift, even if pressure was put on the ram while pumping. (I carried it to the table vertically- ram up- so no air would mix with the oil. I also had the table propped up vertically while installing.)

If I lift the scissors by hand to fully extend the ram, the table will drop back down about 1/2 way and stay in place - even overnight.
I tried loading about 600 pounds on the table and the ram did not lower until I squeezed the release lever. Then it lowered as it should.

If I lift the table by hand and let it settle to where it stops, more pumping does not raise it further.

Back On The Bench:
I removed it from the table, took it to the bench and it would not pump up.
I removed the fluid and inspected it for debris. None was found.
I also tried draining & using ISO 68. The symptoms didn't change. (I didn't try ISO 100)
The manual is mute on which weight of oil to use.

My Summations:
A) No seals are leaking because no oil was under it after not using it for 2 months. No oil is leaking now.
B) It seems that the release system is holding because if the ram is lifted by hand, it held the 600 pounds.
C) The safety is not releasing because a manually lifted table can be loaded with weight.
D) I don't think the seal on the ram's piston is bad because a manually lifted table will stay up overnight.

My Confusion:
A) The pump is suspect because the ram won't lift, but no more air is coming out when purging.
OR...
B) I am also suspecting that I am not purging the air properly. (I even let it sit with the ram extended for a day to see if more air rose out of the system. No luck)
C) The fluid level is OK because the level is at the fill plug opening when the ram is retracted.
D) I have absolutely no idea why, while on the bench, I have to press lightly on the ram to get the pump to raise it.

Attached is a photo of what this hydraulic pump looks like. I apologize that I'm not the best of photographers!

Thank You All for any ideas you can lend to this project. I'm stumped!

Enjoy This Day!
Paul

 

[Bob Korves]

Check valve not seating well or pump seal(s) not quite sealing to the bore when they are at rest.

 

[kvt]

is there any way that the release leaver could be getting opened a little when it gets to a certain point in the travel. I had a jack that did that, and found that once I redid the release stuff it worked again. If you did a bench test on it and it holds up when extended then it should be able to hold when installed unless something is closing all the way, or something.

 

[rgray]

I had a floor jack like that. If not let down all the way it worked fine. If let completely down it would not raise.
Spring loaded ball check valves with a broken spring.

 

[PGB1]

Thanks Bob, Ken & Russ for the helpful ideas. You all saved me on this one!

Based on your suggestions, I got brave & disassembled the cylinder nearly completely for inspection. (I left the main ram in its cylinder.)
For Safety's Sake: Before removing the safety valve screw, I wanted to send the screw fully closed & count the turns it took, but the screw was already 100% clockwise. I counted turns while removing, just to be sure I'd get it back exactly where it was. It went back the same number of turns & is still fully front-seated.

Upon disassembly, I found no thickened sludge or chips, etc. (I had left the originally drained oil in a jar to see if anything settled. It was clean.)
I cleaned every part & the pump bore with lint free cloth and fresh oil & inspected the balls, springs & seals for damage. All looked good to my (poor) eyesight.
I did flush all the tiny passages I could access by forcing oil through with a syringe & long hypodermic needle.

A Side Note:
The pump seal is pretty interesting. I expected to find a larger diameter at the end of the shaft, with an o-ring & a backer or something similar.
The way this one works is that the entire length of the pump bore is lined with rubber and the pump shaft is a simple polished rod with the same diameter its entire length. The downside is that most of this rod is exposed when not pumping, thus subject to picking up dirt & causing abrasion of the rubber lining. (The shaft & outside of the bore can be seen in the photo inside the pump spring.) I'll make a dust sleeve for it tomorrow.

SUCCESS....
After re-assembly & purging the unit worked very well on the bench. I no longer needed to press on the ram to get it to lift while pumping.
Therefore, there must have been something not letting a check valve seat, like Bob suggested, or something was stopping the release valve from fully closing, as Ken mentioned, or one of the springs wasn't working right- maybe cocked & stuck, along Russ' suggestion. My eyesight is quite poor, so I surely missed whatever it was causing the problem during cleaning.

I re-installed the cylinder in the table (never an easy feat) and tested.
At first, it didn't raise & I found it needed about an once of oil. I added oil & repeated purging. (I only can guess that the cylinder is at slightly less of an angle in the table than I had it on the bench, thus requiring more oil.)

After re-purging, it lifted strongly, smoothly & it held.
I stacked about 400 pounds ont he device and the hydraulics still worked very well. Many cycles later, it continued to work & didn't drift down over an hour or so. (Yay!)
I never load it near its rated capacity of 1,000 pounds, so I'm not terribly worried if I somehow disturbed the safety valve's setting by cleaning the balls & springs.

My dog buddy is very happy now, too.
When I get the table out to load my truck with cables & equipment, he runs over & jumps on for the ride up to the truck bed. He then waits until the cart is empty and jumps back on for the ride down & the next load. He takes up table space, but sure has fun! (As do I)

Interestingly, on the bench it took 16 strokes to raise the ram to full extension. In the table, it takes 50 strokes. I'm guessing I still have some air trapped. I'll let it sit overnight and re-purge.

Thank You All again for the great ideas. You really bailed me out on this project!

Enjoy Today!
Paul