Harbor freight currently has the below electric scissor jack on sale for $50 (I think they are also available at WalMart). One probably could use one of these as a linear actuator if one wanted to make a large arm out of 2x4 lumber or similar. Also the bottom screw jacks might have large gearhead motors (or drill motors) attached to make large linear actuators.
They can be had for around $70 if you look in the right places.
There are 2 versions, one with a pot and one without.
Or they have 400 lb push actuators as well that can hold over half a ton.
And only about $85 if you look around.
These are the Firgelli ones but there a number of other manufacturers that sell that exact same thing, just different name.
Even at $100 a piece it will be much cheaper than ordering the scissor jack and other parts plus paying shipping on large heavy items. And you end up with an accurate and powerful actuator already built in a variety of lengths.
I’ve looked at the Firgelli actuators in the past. They would make for a larger arm fairly easily. They also sell the 20a relays used to control them if needed. If one makes a really large arm, then large lead acid auto batterys would probably be used for power, backed by a high capacity battery charger. The real challange that hasn’t been answered yet is an easy and inexpensive way to turn linear actuators, winches, and scissor jacks into really big servos, controlled via controllers like the ssc-32. I’ll have to do some thinking and tinkering on this part.
You can use the guts from an HS-805 servo in one of the actuators. I’ve done it before.
Except you have to run them at 6 Volts so you only get 50 lbs of thrust and half the speed. That is what Servo City did for their R/c actuators. That’s why they are rated at 50lb. I’m not sure how they justify selling a $70 actuator with a servo board in it for $200 but they do.
I’d look at hacking a HS-422 servo for a test project as it is still made with wire connections internally instead of every thing directly soldered to the circuit board. Disconnect the motor in the servo. Put the pot at the joint being moved. If a larger H-bridge is to be used to control the bigger motor, possibly parallel it’s control with the servo’s internal H-bridge control. If motors are larger than what an H-bridge can handle, then use the output that went to the servo motor to operate relays or switch solenoids to control the large motors. The devil is always in the details, but sometimes they actually get solved.
They are not cheap, but then if you were looking at doing it yourself, I think they look like reasonably good value. The 6-12V ones that are ‘comming soon,’ look very nice also.
They also sell them without the servo control, just as actuators with pot, and limit switches if you’d want to play around: servocity.com/html/12v_linear_actuators.html
They look like good quality and good prices. I do like the look of these as well.
why not just use an openservo board? should be cheaper than buying a servo to hack the board from just to wire it up to the actuator. guess it depends on how much current you need the h-bridge to handle. on the other hand if the board is free and moving about then you have an opportunity to heatsink the MOSFETs as well.
Servo’s aren’t really designed to be used as leg actuators either, and there’s reasons the producers of those servo’s don’t provide too much information concerning lifespan
I’d say you’ll probably not get the same lifetime from an actuator used for a legged robot as you would from one used to open a gate 2 times a day, but than that’s two completely different purposes.
All in all, i’d bet the firgelli would last much longer than a decent quality RC servo.
The thing about linear actuators is that once they position themselves, then the motors should turn off until the next move is made. The motor doesn’t have to hold a position like in a normal servo.
That’s the beauty of actuators. And the Firgellis can be purchased directly as a high speed option as well.
They draw less than 2 amps pushing an 80 pound load. Which is pretty good.
I’ve fried HS-322 boards in less than a minute running one. But the HS-805 boards hold up fine. They will even take upwards of 7.2Volts.
That was only the one that I tried, which was the PQ-12 model. It worked okay, but I couldn’t get a decent reading from the internal pot while the motor was running.
I’m sure their closed-loop control actuators work fine…
what do you mean? If someone would pay it they would charge $300 and laugh all the way to the bank. 
