"Contraption" Project: Scratch build motor belt/chain drive w/ variable for/rev speed

[adambowersva]

hmmmmm....have been looking into the leadscrew concept. I'm thinking it may be easily possible to use a screw similar to the one found in a scissor jack and spin it with a high rpm motor pull a piece up its threads thereby pulling the lever down and reversing direction to allow the lever to raise. One question am I still trying to solve on this tangent though is if the leadscrew can be turned reasonably fast enough to cause the 18inch lever travel in a one second time period considering the high gearing ratio that will likely exist due to this "worm type" gear. Whatcha guys think?

 

[adambowersva]

Anymore input from anyone ???

 

[mneary]

Well you know how much power you need, and have several choices for the type of mechanism.

To further refine your design, the next step would be to formalize the mechanical drawings somewhat. If you're not comfortable with mechanical drawing, a photo of a mock-up might inspire someone who can do it.

 

[adambowersva]

This is true. I am currently thinking the screw like that from a scissor jack may be the most simple option. I guess what is still perplexing me is the calculations and such to figure out if that type of mechanism can spin fast enough to cause the necessary amount of force dispalement in the time period necessary and if so, what speed the motor needs to be running at and any necessary gear ratios.

I guess I am a bit worried to buy the wrong motor for fear of wasting money on something I may not b able to use for that purpose.

 

[mneary]

If a leadscrew is rotating at 720 rpm (12 revs/sec) and it has 12 threads per inch, then the nut will move one inch per second. Does this help?

 

[adambowersva]

Yeah, incredibly helpful actually. If it takes 720rpm a second to move it an inch then I would need a motor to be operating at over 13,000 rpms to meet my need for 18in travel in a second or less. That sounds a seriously unreasonable speed to expect within the size and cost limitations I will likely b working in. Don't want to spend over $200 on a motor. That type of astronomical speed requirement is what worried me about worm gears in the first place. A self locking/holding gear/screw is an awesome idea for this contraption, but appears its gonna b killed by speed requirements alone.

 

[Sceadwian]

Lead screws have other resitrictions too, such as the thermal expansion and slop of their gear trains.

 

[Andrew Leigh]

As we are brain storming,

If money was not a limiting factor then a linear pnuematic cylinder would also work a treat.

What about a cam? Excuse my poor drawing but you get the drift. You could make it a double cam like on a car, every 180° you have a low and high point, will only need one direction of rotation? You can dimple the top and bottom of the cam to get a location point.

Cheers
Andrew

 

[Andrew Leigh]

What are you making, agree with the previous poster, will make suggestions much easier.

 

[mneary]

It would be a big cam, since the travel is 18", 40 - 50 lb, negligible inertia.

Although it looks like we're driving a spring through a lever... maybe a cam could apply a greater force at shorter travel.

 

[Gary B]

To reiterate some of what has gone before since my last visit:
1) I suggest a power contactor (heavy duty relay) double pull double throw as the reversing mechanism.
2) Place limit switches to stop the motor before it stalls. Spring load them so the motor cuts off at a certain force instead of a certain position if necessary. Otherwise, just mount it at the full spring compression position.

 

[adambowersva]

It would be a big cam, since the travel is 18", 40 - 50 lb, negligible inertia.

Although it looks like we're driving a spring through a lever... maybe a cam could apply a greater force at shorter travel.

That is absolutely correct. A spring will be driven by the lever. Not sure about a cam....definitely never looked into that as I know nothing about them really unfortunately...would require more research to check into that.

Further, don't know how much it changes things and possibilities but, I have modified the plans...now we are looking at 12" as the travel at most. Weight still remains probably in the 30-40lb range.

And for all those that have posted...Sorry about my delay in getting back. I have been out of town last few days. All of the help you continue giving is phenomenal and greatly appreciated. =)

 

[adambowersva]

To reiterate some of what has gone before since my last visit:
1) I suggest a power contactor (heavy duty relay) double pull double throw as the reversing mechanism.
2) Place limit switches to stop the motor before it stalls. Spring load them so the motor cuts off at a certain force instead of a certain position if necessary. Otherwise, just mount it at the full spring compression position.

Will check into the power contactor Gary Thanks. As for limit switches...I had thought about this before got into the leadscrew/worm gear tangent. Was thinking off the idea of either a plunger type switch that would cut power when pressed by the lever reaching bottom most point or thinking of a mechanism like those built into the electric scooter speed controllers that kill motor power once the brake is applied. =)

 

[adambowersva]

Ok, previously the only viable options for holding the load without power seemed to revolve around some sort of worm gear or leadscrew. Upon continued research I have come upon the products that are used to power a boat anchor up and down such as the one found here...Amazon.com: Powerwinch P77724 Marine 24 Series Pontoon Boat Anchor Windlass Winch: Automotive.

The question is how do these then hold their weighted load when power is not applied? Obviously powering an anchor half way up, the cutting power won't result in the anchor falling back down to the bottom so how do they maintain that weight? As the example given is designed for a 40lb anchor and retrieves at a rate of 80feet/min it may be sufficient to run my contraption so long as I can figure out how to get more control over the speed of the motor. But, I obviously would still like to know how it works in it's load holding and also if there are any ideas as to whether a simple speed controller and throttle would work theoretically to control the motor's speed. Thanks again.

 

[Sceadwian]

Ummm... three words.
Electrically actuated brake.

 

[adambowersva]

Ummm..Constructive helfpful response.

Ok then, I have looked for these and not found a cheap variant yet. A little more help would have been great...

three words....More help please.

 

[Sceadwian]

It would be external to the motor itself, a mechanical assembly that locked the shaft of the motor output, it's that simple. How you implement it is completely up to you. You should be able to find some motors out there with brake assemblies. For examples of how you might go about it simply look up a very basic clutch assembly, and for a brake you simply immobilize one side of the clutch.

 

[adambowersva]

hmmmm...better. Thx, cause I'm definitely no electronics einstein

 

[Sceadwian]

It's more of a mechanical problem not an electronics problem, if you're talking about things like this electronics has very little to do with it. So what you need to do is sit and stare at it long enough thinking about what you're trying to do and coming up with possible solutions until one works. If you can't think of anything you need to mix things up and come at it from a different angle, until you find one that works for you. Just don't keep trying the same thing over and over again with microscopic changes if it's not working, and if you're drawing a complete blank talking it over with someone else even someone that isn't technically apt can help get the creative juices flowing, often simply talking allowed is enough to get the brain working because hearing what your saying lets your brain reprocess things.

 

[Gary B]

A simple brake can be built with a solenoid, spring and paw that engages a gear. Power to the motor also goes to the solenoid that pulls the paw out of the gear’s teeth. When power is removed, the sprig pulls it back into place. To prevent any chance of slippage, use a gear with square cur teeth.