I've used the heck out of this Harbor Freight 6000lb scissor lift, and it has performed flawlessly over the past five years. However, last week one of the rams started leaking and was spewing oil through the cylinder vent hole when being pressurized. So after a good discussion with knowledgeable folks on the GG it was determined that the cylinder seals were probably compromised and it was time for a rebuild.
The lift is a common Chinese import that is sold by many other companies under their own brand names, so most all of the procedures I've listed will likely work on most of the imported scissor lifts on the market. A very common single acting cylinder design (i.e., the ram pressurizes in one direction only and uses the weight of the lift itself to compress back down to its closed position).
First thing to do is remove the cylinder from the lift frame. A simple "E" clip holds the locking pin in place and the pin is easily removed with a punch and hammer. Then the hydraulic line is disconnected from the cylinder and a plug is used on the line fitting to keep the fluid from spilling everywhere. Now the cylinder can be easily removed from the lift.
The next issue is a bit more complicated; how to remove the top locking collar assembly form the cylinder body? The collar itself had three equally spaced holes on the side to accommodate a special wrench. Since I didn't have such a wrench I decided to just stick 1/4" bolts in the holes (they provided a snug fit) and hit the bolts with a large hammer -- SUCCESS!!!
The collar moved 1/4" and I was able to unscrew it the rest of the way with a pipe wrench. An oil drain pan helped in catching all the oil that had seeped past the seals -- there was a LOT of oil in that cylinder that has seeped past the seal.
Once the ram assembly had been removed, I was immediately struck by how much rust had accumulated on the ram components. The cylinder walls were in good shape, but the ram stopping collar, the piston, and a portion of the upper locking collar walls were heavily rusted.
To remove the main oil seal, loosen a simple snap-ring on the bottom of the piston. Then to remove the piston itself, the plastic guide ring has a split in it to make it easily removable. Once removed, the piston locking pin simple pushes out by hand.
The cylinder tube itself was in pretty good shape.
Here are the major internal components ready for inspection and cleaning.
I used a fine wire bench grinder wheel to clean up the rust and scale on both the piston and the stopping collar. They cleaned up very well and there is no real lasting corrosion damage to be seen. I guess I rebuilt this thing just in time -- LOL
The corrosion in the locking collar assembly was a different matter. It was a bit more severe, so I used a die grinder with a small coarse wire brush to clean the bore; being careful not to ruin the wiper seals at the other end of the bore.
A good session of wire wheeling with the die grinder brought the bore back to life -- it looks in excellent shape
With all the components cleaned and ready for reassembly, it was time to look at the seal. The thing is definitely hard and full of bulges, so I can see why it was leaking. Being Chinese, it's metric and the numbers on the seal are as follows: UN 55 70 12
Using a caliper, I established the fact that the ID of the seal is 55mm, the OD is 70mm, and the width is 12mm. I don't have a clue as to UN, but my local hydraulics shop found the seal at their warehouse and it should be ready for pickup tomorrow afternoon.
The locking collar has a vent and it's through this vent that the oil was leaking. It's simply a small filter screen element to keep out debris and is held in place by a locking nut. This area was also corroded and was cleaned out thoroughly with brake cleaner and a small wire brush.
Here are all the ram components cleaned and ready for reassembly once the seal arrives tomorrow.
The most interesting question remains -- why did this cylinder fail and why is there so much corrosion and rust on the internal components ? After thinking about it for a few hours I think I have an answer.
Last year Mrs GF parked her daily driver over this lift most every evening -- summer and winter. In the summer the condensed water from the AC unit dripped down on the lift, while in the winter the snow melt dripped on the lift. Just by luck, this ram was sitting with the vent hole facing up and was on the side of the vehicle where some of the AC condensate and snow melt could potentially drip down into the cylinder vent. Over the course of a year or more, there was enough water going down that vent to cause this corrosion.
This is the original position of the vent hole -- facing up. The other ram has the vent facing down, so no much of a chance of water getting there.
It's as good of an explanation I can come up with, but it makes sense; only moisture could have caused such a huge amount of rust and scale. To remedy the situation, I'm going to install a small elbow fitting in the vent to keep water from directly entering the ram cylinder.
Stay tuned for part II (re-assembly) when the seal arrives tomorrow some time.
