Question about springs to support a "floating" table

[Philip Potts]

I have been searching but I have not been able to find the information I am looking for. I want to build a "floating table" that is supported by one or more compression springs. When an object is added the table will lower by just a bit. As another object is stacked on top of the first one the table will lower a little more and so on until it has bottomed out. Then as an object is removed the table will raise up, etc. I want to keep the height of the stacked objects at a (more or less) constant height so that the person placing the objects will not have to bend over to add or remove the item.

What is the formula to configure the correct spring to accomplish this?

Thanks in advance.

 

[JBA]

Are all objects to be placed on the table the same weight and same height?

 

[jrmichler]

If you want to set an object on the table and have the table lower by the height of the object, your spring needs a "spring rate" equal to the weight of the object divided by the height of the object. Example: A 1" high 10 lb object needs a spring rate of 10 lbs / 1" = 10 lbs/inch. Then measure up the total distance the table has to move, and calculate the weight of the empty table. Then go into a spring catalog start looking for springs. Keep in mind that you can stack springs to reduce the spring rate and increase total travel. You can also use a lever arrangement to use a shorter stiffer spring.

 

[Philip Potts]

JBA: Yes, all the objects are the same weight and height

jrmichler: Thank you so much. With the information you supplied this might be easier than I had hoped.

 

[JBA]

Since the objects are all the same height and weight then the calculation is exactly as described by jrmichler.

 

[Philip Potts]

Keep in mind that you can stack springs to reduce the spring rate and increase total travel.

How so? Wouldn't two 10" springs be the same as one 20" spring? Also, for my project the object weighs three pounds and is eight inches tall. This gives a spring rate of 0.375 pounds/inch. I assume that this would be for one spring. If I were to use four springs, one at each corner, would I divide the rate by four to give a spring rate of 0.09375?

 

[Randy Beikmann]

How so? Wouldn't two 10" springs be the same as one 20" spring? Also, for my project the object weighs three pounds and is eight inches tall. This gives a spring rate of 0.375 pounds/inch. I assume that this would be for one spring. If I were to use four springs, one at each corner, would I divide the rate by four to give a spring rate of 0.09375?

Hi Phillip. If you use 4 springs, one in each corner, they each contact the support and the platform. This is called having "springs in parallel," and their stiffnesses add. So in your case you would want each to be 0.375/4 = 0.09375 lb/in.

As jr said above, if you stack springs on top of each other, you make a softer spring. This configuration is called "springs in series." If the springs are the same stiffness, then two springs in series give you half the rate of each (they see the same force, each compresses the same, and their commpressions add). So two stacked springs of 1 lb/in would act like a single 0.5 lb/in spring.

I like jr's other idea to use a linkage connected to a spring. You could use the linkage to keep your platform from tilting. The spring rates you are talking about are very low, and depending on your task, the platform tilting might interfere with it.

 

[BvU]

for my project the object weighs three pounds and is eight inches tall.

So how many do you plan to stack ?

 

[Philip Potts]

Thank you Randy.
You have given me some very useful information. I am familiar with parallel and series circuits in electrical systems but I did not realize this also applied to springs. I didn't give too much detail about the object because I didn't want to clutter things up. So this seems like a good time to give the details. The object is a stamped metal piece that is shaped somewhat like a U. It is flat across the bottom, 21 inches long, 15 inches wide, and 10 inches deep and weighs 3 pounds. It is open on both ends and tapers from 15 inches at the front to 12 inches at the back. Both sides are splayed outward so the parts can be stacked one on top of the other and placed in a plastic container. The container is then taken to a weld cell where a robot will pick up each piece and weld nuts, bolts and studs on. It then stacks each piece 10 high on a conveyor. When the stack is complete all 10 pieces roll down the conveyor where they wait to be picked up. Then a person brings a small cart, puts the 10 pieces stack (which now weighs 30 pounds and is 11.5 inches tall) on it and rolls the cart to another weld cell where this part is spot welded to another part to make an assembly that will eventually be installed into an automobile. Where the 10 piece stack comes off the conveyor is where I come in. Because production is increasing management has asked me to build 10 more of the small carts. Because things are crowded I came up with the idea I have presented here.

I don't have an actual design yet but what I am thinking is to use four shafts make for linear bearings to act as guides at four corners. Place the springs over the shafts. Build the table and attach linear bearings and place this assembly on the shafts to be supported by the springs. The top of each shaft would be attached in a manner so that each is firmly mounted to prevent sway and tilt. When completed, and assuming everything works as I hope it will, the person who loads the cart will be able to place a stack of 10 on the table, the table will lower by the correct distance, the person will lower a shelf and place another stack. The table will lower again and be at the bottom of it's travel. Then lower another shelf and place a third stack. The reverse will happen when the parts are removed except the pieces are removed one at a timer rather than as a stack. This way I triple the number of pieces on each cart without taking up too much room. I think this will work but it depends on selecting the correct springs.

So again, thank you for the information and I welcome any further insights you might have on this project. And anyone else for that matter.

 

[Randy Beikmann]

Thank you Randy.
You have given me some very useful information. I am familiar with parallel and series circuits in electrical systems but I did not realize this also applied to springs. I didn't give too much detail about the object because I didn't want to clutter things up. So this seems like a good time to give the details. The object is a stamped metal piece that is shaped somewhat like a U. It is flat across the bottom, 21 inches long, 15 inches wide, and 10 inches deep and weighs 3 pounds. It is open on both ends and tapers from 15 inches at the front to 12 inches at the back. Both sides are splayed outward so the parts can be stacked one on top of the other and placed in a plastic container. The container is then taken to a weld cell where a robot will pick up each piece and weld nuts, bolts and studs on. It then stacks each piece 10 high on a conveyor. When the stack is complete all 10 pieces roll down the conveyor where they wait to be picked up. Then a person brings a small cart, puts the 10 pieces stack (which now weighs 30 pounds and is 11.5 inches tall) on it and rolls the cart to another weld cell where this part is spot welded to another part to make an assembly that will eventually be installed into an automobile. Where the 10 piece stack comes off the conveyor is where I come in. Because production is increasing management has asked me to build 10 more of the small carts. Because things are crowded I came up with the idea I have presented here.

I don't have an actual design yet but what I am thinking is to use four shafts make for linear bearings to act as guides at four corners. Place the springs over the shafts. Build the table and attach linear bearings and place this assembly on the shafts to be supported by the springs. The top of each shaft would be attached in a manner so that each is firmly mounted to prevent sway and tilt. When completed, and assuming everything works as I hope it will, the person who loads the cart will be able to place a stack of 10 on the table, the table will lower by the correct distance, the person will lower a shelf and place another stack. The table will lower again and be at the bottom of it's travel. Then lower another shelf and place a third stack. The reverse will happen when the parts are removed except the pieces are removed one at a timer rather than as a stack. This way I triple the number of pieces on each cart without taking up too much room. I think this will work but it depends on selecting the correct springs.

So again, thank you for the information and I welcome any further insights you might have on this project. And anyone else for that matter.

It sounds like the parts are nested, so the stacking height of each part will be less than the actual part height. So for each part added, you'd only want the stack of parts to move downward by a part's stacking height. I couldn't tell if you took this into account.

I did a little searching, because I remembered seeing something similar for cafeteria trays. If you google "tray stacker cafeteria," you may see some designs that give you more ideas.

 

[Philip Potts]

I did a little searching, because I remembered seeing something similar for cafeteria trays. If you google "tray stacker cafeteria," you may see some designs that give you more ideas.

Thanks, Randy, now that you mention it I too have see the trap/plate thing at buffets and such. As I remember an employee would just drop a stack of plates in and then they would rise as each customer took a plate. This is the exact same thing I would like to do.