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69L71
Senior Member




608 posts [100%]
Millersville MD

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 4-Post Lift Collapse - The Probable Cause Reply to this post

After review of the failure that occurred to my Perfect Park 7000 lift on March 15, I believe I have determined the cause. Or should I say causes. The collapse was caused by a dual failure of the actuation system and the locking mechanism in the post. The actuation system failure was diagnosed by the manufacturer, Gemini Auto Lifts, and appears to be right on.

Actuation System:

The actuation system on virtually all residential 4-post lifts is comprised of a hydraulic ram that acts on a series of cables that attach to the top of the posts. The cables run through pulleys, both under the runways and on the crossbars. As I said in my earlier post, I had assumed that I had let one of the cables get slack and that the cable had fallen off the pulley. This was a logical assumption at the time. After consulting Gemini, however, they had a different opinion.

This picture shows one of the pulley-packs under the runway.

A stud is installed up into the runway, and the pulleys are installed with brass bushings and a brass spacer. Then the stud is capped off underneath the runway with a nut, which is secured by either locktite or weld (hard to tell which). Gemini suggested that damage/wear to the bushings could cause galling on the stud/axle. They surmised that friction between the pulley and the axle/stud caused the stud to back out of the runway, allowing the pulley to separate. One of my pictures taken the day after the collapse suggests validity to that theory.

Removing the stud and inspecting it, showed the galling and damage Gemini expected:

Owners and operators of all residential 4-post lifts need to understand that this pulley-pack design is not unique to Perfect Park Lifts. They all have an identical or very similar configuration. After talking with a friend who recently replaced the bushings in his Stinger lift, I advise that all of you inspect your axle studs/bolts and bushings.

Understanding this failure, what prevented the post from locking?

Locking System:

The lock blocks in the posts, combined with spring-loaded lock mechanism are supposed to assure positive engagement of the locks should a malfunction of the actuation system occur. In general it is a well conceived design that should provide plenty of safety. Again, this concept is not unique to the Perfect Park. Perfect Park is unique because of the enclosed slider track, pulleys, cable, and locking mechanism, provided by creating a 3-sided box column. Most others are exposed. The locking mechanism with the column removed is shown below, as is the locking mechanism of a competitorís lift (thanks paso) so you can see how the locks engage.:

With the column installed it looks like this:

As you can imagine, as the lift elevates in its track the lock tangs scrape against the lock blocks and snap into place via spring pressure. When operating the lift, one can hear the locks engage at each stop. If they donít all snap at the same time it suggests that the cables need adjustment. Obviously, it is reasonable to assume that the lift will show signs of wear on the blocks after several uses. This is certainly not a bad thing, and in fact should be seen as a source of confidence. I had my lift for about a year and when I inspected the inside of my columns I saw the telltale wear on the blocks Ė on the 2 rear columns that engaged. The front column (the one that collapsed) showed very little wear on the blocks. (Pics coming later today - check back). The other front column could not be inspected because itís on its side with the lift laying on it. The lack of wear suggests to me that the column that failed had probably not been strongly engaging all along. Obviously, when the lift is lowered onto the lock blocks the tang of the lock would be pulled into the locked condition if there was positive engagement (adequate overlap between the tang and the block). If there was NOT adequate overlap the tang would remain unengaged, and thus, unlocked. This seems to have been what happened in the case of my collapse.

It is unclear to me why the one column showed significantly less wear. I suppose it could be a tolerance buildup, defective spring, or binding in the linkage.

What to do if you own a 4-post lift?

If EITHER the hydraulic/cable system OR the column locks had operated as designed my lift would not have collapsed. Both needed to occur to get the outcome I experienced. That said, I would:

- Inspect your axle studs/bolts and bushings for wear/damage
- Ask your manufacturer and ask if there are inspection or lubrication requirements on the bushings.
- Ask your manufacturer and ask if there are any product improvements that could increase the safety of your lift.
- Inspect your locking mechanism to see that the locks are ALWAYS positively engaging.
- Periodically check your axle studs/bolts (and any other related hardware) for tightness.

This summary is not an indictment of Perfect Park or any other 4-post lift. As Iíve stated before, a lift is a tool, and should be treated with care and respect. My incident, hopefully, will lead to product improvements by the industry and greater awareness by the operator.


[Modified by 69L71, 2:02 PM 4/3/2002]
_______

69 350-350 Burgundy/Saddle Convertible - Loaded (For Sale) Click here for Pics
69 427-435 Black/black coupe, 4spd, 4.11 rear (Under Restoration) Click here for Pics
96' Black/Black LT4

Yeah, I'm the poor schmuck who's lift fell down. Lift Collapse Pics - Click here.
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C4s have that cute little Jennifer Love Hewitt butt while the C5 looks more like J. Lopez

Roger L. Gibbons
Senior Member


109 posts [100%]
Fraser, MI, Macomb MI

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 Re: 4-Post Lift Collapse - The Probable Cause (69L71) Reply to this post

I have been following your posts very carefully, as I have just purchased a four post lift. Obviously I am concerned about what has happened to your lift, can also affect my lift. I am glad that you are keeping us informed as to what was the cause and what can be done to prevent another accident. Thanks for keeping us informed. Roger L. Gibbons
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