Non-Traditional Scissor Lift Shuddering 1

[kgwhipp]

I'm having some troubles with a large scissor lift I custom designed.

The architect managed to build the ceiling height too low to match the distance between the main floor and basement level. If we had used a fixed canopy scissor lift the top would either rest two feet too low when lowered, or smash through the ceiling on the main floor. You can see the problem on the very left with purple and orange bracket dimensions.

Thus, I designed a articulating system to "catch" the canopy on support poles on its way down so it would remain flush with the top floor as the lift fully lowered.

The full up/down cycle worked fine when we had setup the lift in our shop. (Without the extension arms, nor canopy attached). However now, on-site when transition point occurs and the weight of the canopy (approx 4500lb) is removed the whole lift shudders slowly down the rest of the way. It is not a destructive shudder nor a "grinding" but more of a shuffling and shifting around as it lowers.

I initially was positive the problem was with the Counterbalance valves set to the wrong level. These were included in the cylinders as a safety device in case of a line breakage. However much to my chagrin, the lift did the same shuddering on the way UP as well as DOWN. Since our valves should be free flowing on the hydraulic extension stroke, I don't believe the problem is caused by the CB valves and might lay with a fault in the design of the support poles and the UHMW.

Is there a way a CB valve could be causing the problem on the way UP as well? I'm really starting to fear there may be another problem we're missing here, I had wished that it was just a matter of tuning the valves. What are your thoughts?


-Kevin

 

[Compositepro]

Sounds like you have an alignment problem between the guide posts and slides. Why do you have slides at all? The scissor mechanism is designed to do that. You've added a second set of guides that must align perfectly with the first. Your posts should only be stops and not require tight alignment. The spacing at the base of your posts seems to be off since the binding is occurring near there and that is where your posts cannot move.

 

[JohnRBaker]

I can't help but ask, but is it possible that this is what the media was talking about a month or so back when it was reported that as part of his rebuilding/remodeling of his SoCal 'beach' house near La Jolla, that Mitt Romney needed additional parking spaces, however his contractor was not allowed to build a larger garage due to the size of the lot, so he was opting for having a basement put under the garage and adding 'elevators' so that there would be enclosed space for more vehicles than what the original site plan called for?

I ask because this looks exactly like what one would expect something like this to be and since it's being added to a structure which has already been designed, and I assume approved for construction, that this is the sort of situation, with the restrictions mentioned above, that one might find themselves in.

Anyway, sorry if my speculation is way-off the mark, but as I said, I just had to ask ;-)

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[kgwhipp]

This isn't the Mitt Romney house, but we have done some interesting work in the La Jolla area, so I'm surprised I hadn't heard of the car elevator.

CompositePro,

The posts aren't anchored down yet, so the hope would be that they would find alignment when the lift finally reached the bottom. It seems to occur exactly when the weight of the canopy is removed which tells me if the problem is alignment then the canopy is somehow helping alignment.

What's surprising is that the poles don't seem to be lifting from the ground while the lift is shuddering while going up...

I intended for the slides to help support against buckling and to prevent "clanging" if the beams weren't perfectly straight and true. However, I suppose removing them might be one of the few options we have at this point that we could somewhat readily accomplish...

-Kevin

 

[1gibson]

Slip/stick?

The weight of the canopy may be loading the bushings laterally so they "slide" with that load. When you remove the load, they don't have that constant force or friction.

Try bolting the poles down, the slight misalignment you will inevitably have, might also give you some lateral loading and possibly be a cure, if it doesn't bind.

Try lighter lubricant (oil).

Try adding a small weight offset to one side or one corner of the platform, and see if it still happens.

The redundant constraints (poles) make it very likely that not all 4 bushings are in contact simultaneously, and could promote the slip/stick condition.

 

[JohnRBaker]

I assume that the 'canopy', as you call it, is held in place while in the 'down' position by supports around the opening in the floor, correct? If that's the case, why not just let the lower portion simply leave the 'canopy' behind and not be connected to it all once it reaches that point where the 'canopy' is no longer being held-up? What I'm asking is, are the uprights even needed to keep the lower platform level. If the scissors-jack is self-supporting, why not just place four short tapered 'spuds' at the four corners which mates with four 10-foot corner tubing running down from the bottom of the 'canopy'? For the first two feet of upward motion, the lower platform is only lifting itself in a freestanding state, but at the 2-foot level the four corner 'spuds' nest inside the lower open-end of the tubes coming down from the 'canopy' and from the point upward the entire structure is being raised, without any sliding of anything over anything

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[kgwhipp]

Thanks 1gibson,

Good observation, We'll definitely throw a few anchors and see how that affects things.

I had never heard the term stick/slip as a phenomenon but I'm certainly familiar. Now that you mention it, it certainly sounds like a possibility of what's occurring.

Lower friction lubricant also sounds like a worthwhile venture.

It seems like all indications point to problems other than the counterbalance valve. We'll be on the site tomorrow and I'll have to try these out, if anyone else has further thoughts do please share.


-Kevin

 

[IRstuff]

I think it's the difference between having the scissor heavily loading the scissor and unloading the scissor. When unloaded, the scissor is rocking the bottom plate because the canopy constrains the bottom plate from rocking up on the left side and keeps the bottom plate level. Once the canopy is removed, the mechanism which is moving the scissor arms can potentially cause the bottom plate to rock.

That's assuming that the free ends of the scissor aren't constrained by either the base or the bottom plate. A potential way out would be to put the wheels on the free ends in horizontal u-channels in the base and top plate to ensure that the free ends are always in the same plate as the pivot ends on the right side.

TTFN
faq731-376

 

[kgwhipp]

JohnRBaker,

There are actually no supports for the canopy on the surrounding concrete. The top and lower platform have identically sized platforms, thus anything used to support the canopy would interfere with the lower scissor lift platform as it traveled up through the opening. Definitely wish there was an ability to incorporate structure into the concrete but it had already been poured when we arrived.

IrStuff,

I'll take a look to see if the wheels are moving on the rollers, but from what I remember last Friday, the shuddering didn't seem severe enough to indicate a problem there. It's definitely a more of a "sloughing" then a rapid shudder. Big thanks for giving me something else to inspect with more scrutiny.

-Kevin