Electric actuator with multi-start Acme screw

[dlee83]

I am working on an electric actuation standard for my company (we make gate valves). I have found a lot of info on the web about Acme power screws, which are what we use in our actuators, but there's one thing I can't seem to find: a discussion about the availability/practicality of using multi-start screws. I am aware of the mathematical relationship between lead, torque, speed, etc, but I wonder whether it makes sense to recommend anything more than, say, a two-start screw. Even though a three- or four-start screw might make sense theoretically, I wonder if such screws are readily available at a decent cost, or if there are other reasons to avoid specifying them. Any insights? Thanks.

 

[dlee83]

By the way, I'm generally looking at 1"-3" diameter screws.

 

[MikeHalloran]

I can only imagine needing a three or four start screw if you need a valve that moves very, very, fast. Is there a demand for super-fast valves in the sizes you make?



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Considering the amount fo reduction gearing used between the drive motor and the actuator output shaft, the increase in speed caused by a multi-start screw seems to be more easily accomplished by modifying the reduction gearing. I guess your actuator is something similar to the 'Rotork' pattern.


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If we learn from our mistakes I'm getting a great education!

 

[JimCasey]

A multi-lead screw will necessarily be steeper,and it will progressive lose the self-locking effect. So any thrust against the drive nut will tend to back-drive the actuator. That'll make you need a mechanical brake to keep from overheating the motor. Also your mathematical tools should reflect that the efficiency drops off rapidly. This effect also will cause the motor to work harder for a given distance or force.

 

[dlee83]

I think all of your points are good. This confirms my suspicion that only in extraordinary circumstances would one use a three- or four-start screw. Thanks for the input.

 

[gerhardl]

dlee83,

it might be interesting for you to have a look at how a world-wide European company solves this problem, several electronically controlled versions and options available.

http://www.auma.com/cms/auma/en/

 

[reverman]

I have a formula I use to determine self locking threads. It's pretty conservative but it hasn't failed 40+ years:

0.079 >= X

X = Lead/(PI * Mean Thread Diameter): this is the tangent thread helix angle

Lead: for example if you have a 3 lead thread and a 1/4 Pitch the lead = 0.75"

PI = 3.14...

MTD: (Nominal Thread O.D. - 1/2 Pitch) : 2.5" diameter thread with a 1/4 pitch would be 2.375"

so for our above example (0.75/PI*2.375) = .100 so this thread would not be self locking

The coef. of friction of 0.079 is the conservative portion (the value came from a motor manufacture) since when calculating the stem torque we typically use a value of 0.2

Ref: "Mechanical Engineering Design" 2nd Edition PP295-299

 

[JimCasey]

If you need a faster actuator there are Pneumatics and hydraulics. Fast electrics exist, but tend to be very small and specialized.