Guided screw actuator

[Open to Advice]

Hello, I am in need of some direction or advice. We have built a very simple 1x2 retangle tube 30 inches long that goes from vertical to horizontal. The load on the arm is 250lbs. I prefer to use a rodless acme screw because of space constraints to lift and lower this arm. The problem I am running into is everything I am finding is for very accurate movement. I am thinking more like the design on a screw drive garage door opener. I simply want to lift and lower it 10-20 times a day reliably. Does anybody know of such a screw?

[TomB]

I don't know what a guided screw actuator is but I have a compound boom for lifting and mounting heavy assemblies that is actuated by a 5/8" 'all thread' rod with a hand crank. It works quite well but the movement is from about -135 to -45 degrees which is not the same as lifting. (The compound boom extends up 5 feet then has a second dangling boom that must swing in and out to put the assembly on/off an axle.)

But that said I expect you need to provide more detail before anyone can give more specific advice. The load might be lifted by pushing or pulling and that would change the design options. Depending upon where on the lever you intend to mount the actuator you change the load on the screew substanitally. Whether you want a motor or hand crank changes the options. I'm sure many could help if they could better understand your need.

Tom

[BillTodd]

Do you mean a rotary actuator ? I.e. one that will twist the arm through 90' from the pivot point.

Bill

[Open to Advice]

Guy's thanks for the reply. My application is similar to Tom's. I have a boom (30 in) attatched to a wall that has a travel of 90 degrees (vertical to straight out). Attached to the end of this boom is a device that weighs 250 lbs. Because of space requirements I can't mount a actuator under the boom, so what I'm looking to do is mount a motorized acme thread vertically behind the boom and pull the boom via a rod 7" out from the pivot point of the boom so I have tension on the screw that I need to address. This boom will be used tops 20 times a day and I'm looking at 5-6 seconds travel speed. I do want want a smooth operation and with such a low cycle rate I'm thinking a acme crew with bronze nut will accomplish what I need to do. Again thanks for your imput.

Kevin

[Al_The_Man]

Can you use a standard Linear Actuator of the right length?
http://images.google.ca/images?q=lin...ed=0CDYQsAQwAw
There many different types to be had on ebay, also Nook have motorized screw Jacks that come in acme and ball screw.
Al.

[TomB]

Al gave you some pictures to look at but you will have to think about the torque needed. 250# at 30" is a 625 ft# moment. If you are actuating that from 7" out the drive must put out 30/7 * 250# = 1071#. That is more than the force exerted by the sampling of Al's pictures that I looked at but I did not look at them all.

I don't know what price you want to hit but I think there may be some lower cost options. Bill Todd mentioned a rotary actuator. I think I would look into a worm and gear type actuator. Put the gear on the boom and then rotate the worm. It generates lots of torque by using a high gear ratio so a small motor is viable and its inherently self locking. If I wanted to be really cost effective I'd probably hook a 1/4 inch electric drill to it. I'd also consider a cable type winch with a crank. There are some pretty cheap units at Harbor Freight and you could pull at an angle back to the wall with a double, triple or quad bite.

If you continue to think about do-it-yourself screw pulls you will have to consider how you hold the rod up in the air. At the top you will need easy rotation and a 2000 to 6000 pound thrust bearing. (i.e. At a minimum a factor of safety of 2 but that is not adequate if the failure of that bearing could hurt the operator. If it could hit somebody as a result of a failure and crash then a factor of safety of 6 is more appropriate.)

Tom

[Mike Everman]

How about a 12v cable winch at the floor, run the cable up the wall to a pulley above your pivot, then out to the end of your boom? Cheap and quick.

[PrecisionSpindl]

Depending on how you want to lift and lower the arm, a linear actuator may be an economical solution. With a lead screw or a ball screw, a motor and a track all mounted in an assembly, they are convenient. They get used for opening and closing safety curtains and doors, raising and lowering lifts and mower decks. Often times, a linear actuator can be purchased for a little more than the cost of a lead screw or ball screw and rarely are they worth repairing. There are plenty of manufacturers and distributors to chose from.

David

Precision Spindle & Accessories Inc.
www.precisionspindleinc.com tel. 519 671 3911
haviland@precisionspindleinc.com fax. 519 652 5994

[Open to Advice]

Sorry for the slow reply, traveling this week was rough. Tom I agree with your numbers. I have a depth of 4 inches of space for this to work in so I need to either get more leverage or look at another means to lift it. I need this to be producable it's not a one off project. I have given thought to 2 strand roller chain attached to the boom, looped over 2 sprockets mounted 18" apart vertically and stringing the chain in a "S" shape with the end of the chain attatching to a lead screw. After I catch up on sleep I will see if this still sounds like a possibility. Dave based on your post it sounds like you have experience in this area. Would it help for me to post some drawings showing what I'm working with?

Again thanks for the input

Kevin