Considerations for design of lead screw + guide rail set up

[Ivant001]

TL;DR Summary

Seeking suggestions for design considerations for lead screw + guide rail set up to move 400 kg load

Hi everyone!

I'm a mechanical engineering undergraduate and am designing a linear motion system for a school project.

I hope to be able to use this system to achieve opposing motion for 2 x 200 kg loads (i.e. move them closer/ further apart simultaneously). Referring to the picture below:

I'm thinking of using:

• a twin lead screw to move the loads when rotated
• a crank handle, attached to the lead screw, which will be rotated by hand
• guide rails, possibly with bearings, to take the weight of the loads and ensure the loads move in a straight line

I've come up with the following considerations to account for when sizing the guide rails and lead screw:

Guide rails

• Sized to withstand the weight of the loads (compressive stress must not exceed compressive yield strength of material)
• Use ball bearings to reduce friction when loads are sliding

Lead screw

• Sized to withstand compressive and tensile stresses associated with moving the loads (which will arise from frictional forces between the load and the guide rails)
• Sized to ensure that a person can supply sufficient torque to turn the lead screw manually, with the help of a crank. A simple gear assembly may also be required

Am I on the right track? Also, does anyone have further suggestions as to what I should account for when:

• Designing such a system, in general
• Sizing the guide rails and twin lead screw?

Any help would be appreciated. Thank you for taking the time to read my post!

 

[Baluncore]

Welcome to PF.

Am I on the right track? Also, does anyone have further suggestions as to what I should account for when:

• Designing such a system, in general
• Sizing the guide rails and twin lead screw?

Yes you are on the right track.
But you give no 200 kg block dimensions, or the range of separation required.
Does the screw, or do the blocks need to be locked in position?
Is the twin screw under compression or in tension while positioned? What axial forces?
How often will the blocks be moved?

You will need to fix the axial position of the twin leadscrew. The minimum would be to have a deep groove ball bearing, or two thrust bearings, at the handwheel end. The twin screw will be partially supported in the two nuts.

How accurately must the heavy blocks be aligned? That will decide bearing block positions.
Will there be swarf or other contaminants present that could foul or damage the bearing blocks.

You will need two “Fully Supported Linear Rails” and eight "recirculating ball bearing blocks". The rails will require a rigid chassis that also supports the leadscrew bearing.

It would help to know the application of the device with two heavy blocks.

There are situations where you might use a pumped air-table to support the blocks while they were moving. That would blow dust out of the bearing surface. The blocks would lock in position when the compressed air was turned off. The guides would then position, but not support the blocks.

 

[Ranger Mike]

good info
https://www.mcmaster.com/linear-mot...s-3/ball-bearing-carriages-and-guide-rails-9/

 

[jrmichler]

When you are looking for ideas, an excellent resource is the McMaster-Carr catalog (www.mcmaster.com). One way to get equal and opposite motion is by fastening a left hand threaded screw to a right hand threaded screw. One crank handle with no gears. McMaster-Carr has left hand threaded rod and nuts.

 

[berkeman]

good info
https://www.mcmaster.com/linear-mot...s-3/ball-bearing-carriages-and-guide-rails-9/

When you are looking for ideas, an excellent resource is the McMaster-Carr catalog (www.mcmaster.com). One way to get equal and opposite motion is by fastening a left hand threaded screw to a right hand threaded screw. One crank handle with no gears. McMaster-Carr has left hand threaded rod and nuts.

Great minds think alike!

 

[jrmichler]

Great engineers read the McMaster-Carr catalog from cover to cover to learn what sort of stuff is out there, and to get ideas. All 4061 pages of mechanical goodness.

 

[Baluncore]

One way to get equal and opposite motion is by fastening a left hand threaded screw to a right hand threaded screw.

Great minds think alike!

Twin-lead screws are manufactured with a left and right hand thread for dual and opposing motion.
You can make your own by coupling a left and a right, or you can buy available product.
Google images, with the double quotes; “twin lead screw”

 

[Ivant001]

Hi all,

Thank you for the very helpful insights! I’ve taken a look at the McMaster catalogue and it seems to be a very useful resource.

Instead of having recirculating bearings, I’m thinking of using the ball/roller carriages that McMaster has. I am also aware that there are other types of rails and slide mechanisms , such as C-channels.

From your experience, which are the best “carriage mechanisms” for use for high loads in corrosive environments (but no swarf)?

I’ve also thought about how the blocks can be stopped, as suggested by @Baluncore. I’d like to apply a stopper that can stop the lead screws and by extension the sliding blocks. Something like this stop plate by Misumi. I should find out how effective this would be, considering that the static load on the sliding blocks and lead screws is expected to increase (more loads will be placed on the sliding blocks once they are positioned).

Does anyone have suggestions for a stopper?

Also, at the hand wheel end, I’m thinking of including lead screw end support blocks, with a gear system and hand wheel integrated within. The end result is that hand torque will be used to turn the gears and the lead screws.

Do you think this would be possible? Also, does anyone have recommendations for sizing the hand wheel and gear ratio of the gear assembly so that they can be rotated by human torque? I’m thinking I could reference existing hand wheel mechanisms.

@Baluncore What do you mean by how accurately do the sliding blocks have to be aligned? How will this affect the positioning of the lead screw end support blocks? For the application, I don’t think we will be looking at decimal level precision, but the sliding blocks must be able to take heavy loads reliably without being affected much.

 

[Baluncore]

You have given insufficient information for us to know what will be important.

You did not tell us that the screw would sag because the rails were over 6 metres long.

200 kg blocks, sliding blocks, or end support blocks? It gets confusing when there are so many different blocks.

but the sliding blocks must be able to take heavy loads reliably without being affected much.

How much is much?
Is the heavy load in addition to the 200 kg, or is the maximum 200 kg?

Are those loads impact or static ? Are they vertical, aligned with, or across the rails?

Also, does anyone have recommendations for sizing the hand wheel and gear ratio of the gear assembly so that they can be rotated by human torque?

Hand wheel diameter and gear ratio can be ignored until the mass, distance, and the time available to move the blocks has been specified.

Does anyone have suggestions for a stopper?

Yes, It should not be possible to engage the stopper while the load is still moving, or something may break rather than brake. What will happen if the blocks are traveling at speed when they collide, or reach the ends of the rails?