Mechanical Caster Lift

I have a question that maybe someone out there has a tip to solve.

I've used a mechanical caster lift a few different times, but mostly with slightly heavier set pieces that needed to be fairly grounded once rolled into position (so too heavy for good old wagon brakes). I'm talking about the method of casters mounted to a hinged board that when pressed down raises a unit onto the wheels.

My issue is how much I feel like I stress the framing when lowering the wheels. Is it just simply impossible to have the framing of a unit stay square when using this method? If I frame the unit comply solid and square the hinged boards either get themselves wedged (like need to hit the middle hinge with a hammer to free them) or there isn't enough travel for the wheels to clear the floor (without climbing under and swiveling the wheels at least 90 degrees).

I've compensated in the past by just accepting that the unit will need to bow out slightly when up on wheels, and when seated it will be square. This usually means allowing the areas where the hinge boards attach at least some VERY slight movement so things don't get wedge and the unit can exert enough force to get off the casters.

Just wondering if there are tips for this, or if maybe this just happens with swivel casters and there really isn't anything you can do?

Just use a vertical piston with a castor on it. Attach a pneumatic piston to the underside of your wagon. Attach a castor to the bottom of that. Include appropriate tubing, tanks, and regulators and up and down we go.

Without some specifics, its hard to know, but sounds like the span between the support points when lifted is too great - like the full width or length of the unit. Figure out a way to hinge in center to both sides perhaps rather than one edge to the other. Or stiffen the frame on the piece so it doesn't deflect edge to edge - but that can be an endless or expensive rathole.

Sorry about not being clear.

I use the three hinge style lift (one hinge in the middle underneath, two on the outer edges). Lever board crosses middle hinge and you apply pressure down to lower casters and lift unit by flattening the board. Weight of unit lowers itself when you release pressure on the lever board.

Looks like this: https://www.controlbooth.com/attachments/2-mechanical-caster-lifts-4x8-pdf.8919/

Now the few times I've used this method I've used it on set pieces that were moved either hidden or at a time when crew could be seen. The reason is that the swivel casters always seem (at least 1-2 of them) to retain slight weight on them, and someone always needs to turn them to get the unit to rest flat.

I guess I'm just missing something because this issue seems to make sense due to geometry. The shortest distance between two points is a straight line, which means when the caster boards hinge upwards there is only so far they can go (the three hinges tying themselves together and to the outer framing). This apparently doesn't leave enough space for the casters to fully clear.

I'm just wondering if I'm missing something in my construction of these to avoid this issue. I would like to use this method to move a set piece in view that will then be stood on, so I don't want to have to have someone crawl under to kick the wheels out of the way. Yes I could go the pneumatic route, but this is much cheaper and would like to pursue this option first.

Any thoughts?

Back in the day I seem to remember three ways of doing this. None of them are the ones in your diagram. Try one of these.

( Apologies for the hand drawn back of the napkin images )

The closest one ( this from my old parker and Smith book ) to what you are doing.



Looks some thing like this.
Note that there is one major difference between this and your drawing. In my sketch the hinges are NOT in the same plane. ( And one hinge IS taking the weight of the platform). This means that the casters can move higher without binding when the jack is engaged. If they don't move high enough - move them a bit toward the center ( but makes things tippy) and / or make the stacked board on the right side thicker.

The second way was this.


Use a single jack for each wheel. Have some method of lowering and locking them. This was mostly only useful if we ware going to extend the lever arm out of each end of the platform.

The third method worked well for lightweight things ( think a bench )


For this one the stagehand would step on the lever arm ( right side on the picture) which would force up the left side. Then he would pull the stick down ( which raised the right side against the castor plank as he pulled it down)

.

I like the idea of getting the hinges out of plane to allow more movement.

JChenault said:

If they don't move high enough - move them a bit toward the center ( but makes things tippy)

Click to expand...

In addition to the platform being more tippy, moving the casters towards the center would decrease the mechanical advantage of the lever.

Annothre way to avoid the need to turn the casters is to use fixed casters instead of swivel on on side and ensure that the final movement before lowering the platform is one that will turn the swivel casters in the direction they need to go. This does significantly reduce the manuverability of the rolling platform though, so it might introduce new problems to solve.

Although the OP's design states that "Hinges NEVER bear the unit weight"; I believe that they are actually weight bearing (at least in part) while the platform is being raised/lowered between standing on its own and resting on the casters.

robartsd said:

I like the idea of getting the hinges out of plane to allow more movement.


In addition to the platform being more tippy, moving the casters towards the center would decrease the mechanical advantage of the lever.

Annothre way to avoid the need to turn the casters is to use fixed casters instead of swivel on on side and ensure that the final movement before lowering the platform is one that will turn the swivel casters in the direction they need to go. This does significantly reduce the manuverability of the rolling platform though, so it might introduce new problems to solve.

Although the OP's design states that "Hinges NEVER bear the unit weight"; I believe that they are actually weight bearing (at least in part) while the platform is being raised/lowered between standing on its own and resting on the casters.

Click to expand...

The hinges don't take the weight, the caster board is positioned specifically to transfer the weight from the caster, through the board, into the platform frame. Imagine a stone next to a log. Insert a lever over the stone and under the log and push down. This is the same mechanism. All the hinges do in this situation is leave that lever arm in place when the weight is taken off. You can in fact use the entire mechanism with only the middle hinge, which is actually how I put them in place before attaching the outer hinges, because then I know the weight all on the wood and not the hinge fasteners.

BSchend said:

The hinges don't take the weight, the caster board is positioned specifically to transfer the weight from the caster, through the board, into the platform frame. Imagine a stone next to a log. Insert a lever over the stone and under the log and push down. This is the same mechanism. All the hinges do in this situation is leave that lever arm in place when the weight is taken off. You can in fact use the entire mechanism with only the middle hinge, which is actually how I put them in place before attaching the outer hinges, because then I know the weight all on the wood and not the hinge fasteners.

Click to expand...

Yes, I understand this can function without the hardware hinge as you describe. However, once you put the metal hinges in, during the transfer between the up and down positions, the stiffest path for the load to follow from the platform framing to the caster board is throught the hinge hardware instead of the hindged formed by the corner of one board bearing against the other. In redundent structural systems, the distribution of force is proportional to stiffness. While redundency means this distinction may not be important from a safety point of view, it still may be important in selecting hinges from a durability point of veiw. If you think, "this hinge NEVER bears the weight, I can use a cheap little weak hinge here," you'll probably be replacing the hinge soon.