Buttress vs stub ACME thread for high-helix multi-start 1 direction load

[SpeedySpud]

Hey everyone, I’m working on a school project designing a component for our team sail boat and am asking for some input on the design of a screw section:

• Details:

o Lead screw is used to compress compression spring only​

o Axial force requirement not yet calculated but will be very low and in 1 direction only (reaction force of compression spring)​

o User rotates lead nut to create axial movement in lead screw and compression spring used to return lead screw to home position​

o Will be a high-helix multi-start thread​

• Constraints:

o High helix: lead must be ~60mm (2.36")​

o Multi-start: 8 start looks the part but this is not following standard for root/crest width. Standard would result in 12 start but since this is low axial load I will opt for lower count​

o Size: external thread major diameter = ~33mm (1.30")​

o Spatial: thread depth must be as shallow as possible due to need for large lead screw shaft hole diameter and radial clearance limitations. This will likely require us to have a thread depth shallower than standard.​

• Questions:

o Buttress vs stub ACME for high-helix (60mm) multi-start (12) threads:​

 Is this possible for both thread styles?​

 Which is better suited?​

 Which is easier to manufacture with these constraints?​

o Buttress vs ACME for shallow tooth height:​

 Design standards have tooth height at 0.3P for stub ACME and 0.66P for Buttress, but for our constraints (1 direction load) shouldn’t a buttress thread with a tooth height of 0.3P be stronger than a stub ACME if axial load is the same between them?​

Any help is much appreciated, thank you very much!

 

[3DDave]

A ratchet mechanism is cheaper and easier to procure, but you have designed yourself into a corner.

If the loads are low, why care about which is stronger. Care more about where the mating nut will come from because neither is "easier to manufacture" particularly if you are going to "have a thread depth shallower than standard."

 

[SpeedySpud]

Hey 3DDave, thanks for your reply!

I'm not sure if a ratchet mechanism can be used in this situation as both the lead screw and nut need to return to the same (indexed to each-other) home position, but I will do some research.

To give some more clarification to it's function:
1) The user will rotate the screw nut by hand, causing the lead screw to travel axially. The screw nut is constrained axially but not rotationally, the lead screw is constrained rotationally but not axially. Opposite of what is conventionally done.
2) The movement of the lead screw compresses the compression spring
3) When the user is finished rotating the screw nut, the compression spring forces both the lead screw and screw nut to their original home position

The issue is that both screw nut and lead screw must maintain their indexed/clocked home position relative to each-other.

I misspoke when I suggested that loads are not significant. One of our constraints is to have the tooth depth as shallow as possible which is why load capacity of the 2 threads considered is important.

 

[3DDave]

Pick one. If it fails, use the other. Or get both to save time. You have already indicated that you have no other choice.