Irreversible screw drive system

[aerosandy]

Hello,

I am trying to design an irreversible actuator system for trim assembly in small jets. I'am having problems with the present ACME threads, for linear motion. So we are designing it with a thread system with 4 leads. When I say 4 leads, its got 4 different threads starting on the same shaft, this increases the pitch on each threads and also makes the system irreversible (hope everyone can visualise this!). This drive system actually works real good, when compared to the ACME thread system.

My question here is, does anyone here know more about the vendors for these kind of screw drive system or actuator system. Coz am not able to get any search for these kind of threads. And also is there a place on the web where I can get more details about different screw drive system.

Thanks,

Sandy

 

[MikeHalloran]

I understand what a 4-lead thread is.

What I don't understand is how it can be irreversible, given the increased lead for a given pitch, unless it's very badly made.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Mike is right. General rule for lead screws is smaller pitch makes the screw harder to backdrive.

Even an non-backdriving screw may backdrive under vibration.

For an aircraft I would suggest a mechanical brake.

 

[BillPSU]

Making Acme threads on the typical slender shafts used in aircraft is very difficult. If you single point thread the shaft the shaft will either try to climb on top of the threading tool or the shaft will flex away from the threading tool. Use of a follower rest to stabilize the shaft will be necessary. Typically this is done using the best manual machinist/toolmaker you can find.

The other approach would be to have the thread ground which is what I would recommend to you. There are thread grinding shops which will make whatever you want for a price.

 

[DesignerMike]

I have used these guys for linear motion work. They have a lot of info on their site, and good prices on standard products.

Their engineering staff seemed pretty good, but custom stuff drove the price up pretty fast.

http://www.nookind.com/

 

[Barry1961]

http://www.kerkmotion.com/

These people make small light weight multi start high lead screws. They also have the spring loaded anti-backlash units which MAY not back drive if tight. Of course the more you pre-load the nut the harder it is to turn.

I used to work at a place that had an old dual screw, high speed design using these. The payload was about 100 lb and speed around 16 in-sec. They would run good until they got dirty then get hot and melt the plastic nut. Probably would last 6 months to a year in a two shift line.

With your application being slow they should hold up well.

Barry1961

 

[tbuelna]

aerosandy:

MintJulep is correct. Acme screws can backdrive under vibration. They are also very inefficient transmitters of power. That is why they are not typically used in high performance mechanical actuators.

A more typical design would use a ball screw and a fail safe mechanical brake on the drive motor.

http://www.actuation.goodrich.com/flight/actuation.shtml

Regards,
Terry

 

[JimCasey]

tbuelna: I don't think you WANT efficient power transmission in this case. He is driving a trim tab. The motor will be jogged for a very short amount of time then he wants it to stop immediately and stay there.

The ACME thread is good because it is reliable for high-cyclic applications, and reasonably self-locking, and the idea of a brake is excellent.

I am envisioning a simple mechanism of a small motor with a worm on the pinion, driving at right angles to a thrust nut with internal ACME threads, and the output shaft driving a 2-force link to the trim tab. The worm would be self-locking, especially in combination with the nearly self-locking ACME thread.