Any Physicists Or Engineers In The House?

[MaxwellsDemon]

I've assembled a small... Winch... for this photo rig I'm building. It's made up of a small toy motor geared down to about 60 rpm, and it's being used to pull a 5-6 pound load, which is mounted on a set of inline skate wheels + bearings.

I was worried about the tiny motor's torque, but it manages to rotate at a smooth consistent speed.

However, the load it's pulling doesn't move steadily. The string goes taut and then the rig slides a few inches, then stops as the string goes slack. The string goes taut again, and the rig rolls over a bit more, then stops. It kind of bounces with the string.

Steady consistent speed is crucial for this, and this isn't working.

The motor is clamped to a heavier, stationary object. I only tested this on smooth tile. I tried several kinds of string/rope but settled on sewing thread since it was the least elastic and didn't tangle/wind up in itself.

Any ideas on how to keep the motion steady? I imagine applying some sort of counterforce but the bearings and skate wheels slide very easily...

Halp!

[Shot]

Viagra will fix the small winch

[overherebc]

For the sake of experimentation try pulling it up a slight incline. If iti pulls steadily then you know that your motor system isn't getting wound up and released. This might leave you with the chance to do more experiments on your wheels/trolley. Maybe increase the rolling resistance?

[harrry]

My guess is that you have to fear the motor so that it turns at high revs and pulls the load slowly. you could try pulleys instead of gears.

[onionluke]

you could try a line attached below the camera that is weighted or released from another drum at the same speed as the one doing the lifting

[sirineou]

your problem is with the elasticity in the string,

Replase it with flexible wire such the type used to hung pictures.

[thaimite]

To get the load moving you have to overcome the initial friction. Then as stated above the elasticity in the string will cause the load to jump and then it will stop again until the string is taught once again.

You need a nylon (fishing line or similar) in place of the string. It would also help if the load was on a small cart or something with wheels that did not have as much friction if this is possible.

[MaxwellsDemon]

For the sake of experimentation try pulling it up a slight incline. If iti pulls steadily then you know that your motor system isn't getting wound up and released. This might leave you with the chance to do more experiments on your wheels/trolley. Maybe increase the rolling resistance?

This is what I thought too, introducing gravity as a counterweight. However I need to find a smooth surface at an angle... Not so easy so far. I'll be sure to test it though.

My guess is that you have to fear the motor so that it turns at high revs and pulls the load slowly. you could try pulleys instead of gears.

What do you mean by pulleys? How would you change the speed/torque ratio with pulleys? Currently I'm using some sort of 203:1 gear ratio system.

To get the load moving you have to overcome the initial friction. Then as stated above the elasticity in the string will cause the load to jump and then it will stop again until the string is taught once again.

You need a nylon (fishing line or similar) in place of the string. It would also help if the load was on a small cart or something with wheels that did not have as much friction if this is possible.

I'll try and get some nylon... Dunno where though. Any ideas? The load is already on a close to frictionless setup - the bearings provide almost no resistance.

[KeyserSoze01]

Fishing line will work. Although it will stretch also, but not as much as string. Another way to prevent the bouncing of the fixture due to the elasticity of the line is to place a drag-load on the fixture. This way there is a greater load the motor has to pull, but will overcome the stretching on the line as there is a constant load on the line.

[jcgodber]

add a small spring loaded pully to use as a tensioner to your rig

[harrry]

For the sake of experimentation try pulling it up a slight incline. If iti pulls steadily then you know that your motor system isn't getting wound up and released. This might leave you with the chance to do more experiments on your wheels/trolley. Maybe increase the rolling resistance?

This is what I thought too, introducing gravity as a counterweight. However I need to find a smooth surface at an angle... Not so easy so far. I'll be sure to test it though.

My guess is that you have to fear the motor so that it turns at high revs and pulls the load slowly. you could try pulleys instead of gears.

What do you mean by pulleys? How would you change the speed/torque ratio with pulleys? Currently I'm using some sort of 203:1 gear ratio system.

To get the load moving you have to overcome the initial friction. Then as stated above the elasticity in the string will cause the load to jump and then it will stop again until the string is taught once again.

You need a nylon (fishing line or similar) in place of the string. It would also help if the load was on a small cart or something with wheels that did not have as much friction if this is possible.

I'll try and get some nylon... Dunno where though. Any ideas? The load is already on a close to frictionless setup - the bearings provide almost no resistance.

http://en.wikipedia.org/wiki/Block_and_tackle

[overherebc]

For the sake of experimentation try pulling it up a slight incline. If iti pulls steadily then you know that your motor system isn't getting wound up and released. This might leave you with the chance to do more experiments on your wheels/trolley. Maybe increase the rolling resistance?

This is what I thought too, introducing gravity as a counterweight. However I need to find a smooth surface at an angle... Not so easy so far. I'll be sure to test it though.

My guess is that you have to fear the motor so that it turns at high revs and pulls the load slowly. you could try pulleys instead of gears.

What do you mean by pulleys? How would you change the speed/torque ratio with pulleys? Currently I'm using some sort of 203:1 gear ratio system.

To get the load moving you have to overcome the initial friction. Then as stated above the elasticity in the string will cause the load to jump and then it will stop again until the string is taught once again.

You need a nylon (fishing line or similar) in place of the string. It would also help if the load was on a small cart or something with wheels that did not have as much friction if this is possible.

I'll try and get some nylon... Dunno where though. Any ideas? The load is already on a close to frictionless setup - the bearings provide almost no resistance.

Don't know if you can get them in Thailand but the very soft tires used on model racing cars might produce good rolling resistance at slow speed.

[harrry]

You could try putting a sort of brake on the skate wheels to make them roll harder... sort of a block attached to the frame with an arm dragging on the tyre.

[impulse]

Sounds like you're experiencing the difference between static friction and dynamic friction.

Static friction is what keeps it from starting to roll, and it's usually higher than dynamic friction, which is what makes it slow down once it starts rolling.

You're storing up energy in the form of string stretch that releases itself once the contraption overcomes static friction.

The guys suggesting an alternative to string have part of the right answer, though fishing line will stretch 30% or so before breaking. Use a heavier line, go to a braided fishing line, or (better still) use steel wire like they sometimes make necklaces out of.

If it were me, I'd pack the wheel bearings with a very heavy grease to increase the dynamic friction where it's about the same or more than the static friction. You really don't want a free rolling, frictionless system, but you do want a smooth rolling system. Heavy grease will do that for you, though you may have a hard time packing it into the bearings if they are sealed.

If you want to get serious, take one of the shields off each of the ball bearings and pack them full of axle grease.

[arthurwait]

I can't quite picture what you are doing , but can you use steel or nylon wire available in most hardware shops, homepro etc.

Or a rack and pinion or worm gear system. You maybe able to botch a cheap one together with a threaded rod and nut. Nut joined to the truck.

Or maybe stick some foam or rubber around the wheels.

[overherebc]

I can't quite picture what you are doing , but can you use steel or nylon wire available in most hardware shops, homepro etc.

Or a rack and pinion or worm gear system. You maybe able to botch a cheap one together with a threaded rod and nut. Nut joined to the truck.

Or maybe stick some foam or rubber around the wheels.

Think your last line is a good one. foam rubber maybe an 1/8 of an inch thick cut into strips and superglued like a tire on the wheels

[thaimite]

I am sure you have worked out by now that your problem is the difference between the static friction and the rolling friction. There is a build up of energy as you try to overcome the static friction which cause the load to jump when it starts to move and the much smaller rolling friction is what you have to overcome to keep the load moving,

Any stretch in the string or flex in the gearbox will be released at this stage. If the load jumps too far then everything will go slack, the load will stop and you have to start all over again to overcome the static friction.

If you have a poorly constructed gearbox it will also make things worse as the shafts bend or flex as the load is applied.

You should concentrate your efforts on making sure there is as little 'give' in the system as possible and if there is some way of helping it start by a small push on the load,, that will also help.

As it is the difference between the static friction and the rolling friction that is the root cause of the issue, increasing the rolling friction will also be as good as decreasing the static friction although of course the system will be less efficient overall.

[1BADDAT]

The easiest way to add resistance to the object being towed is to tighten the nut that holds the wheel on the axle. But before doing that you might want to dissasemble on wheel to see if there is a spacer between the bearings. If there is a spacer there remove it. This will allow the bearings to be loaded when the nut is tightened. This drag load can be adjusted by tightening or loosening the nut to get the desired result. I am pretty sure this will do the trick.

[thaimite]

Is the surface your load is being pulled upon level? If there is a slight incline it would be better, but if downhill it would be worse.

Another idea if possible would be to have the load as multiple small carriages like a train so that it is applied gradually. As the system starts the small delay before each carriage starts to move should reduce / stop the jumping

[overherebc]

I've gone back through all the posts and putting them all together would suggest lay the thin foam on the surface your wheels are running on.. Add the rolling resistance externally and you don't need to mess about with the wheels and or motor.

[VocalNeal]

The line pulling the load needs to be one continuous loop also connected to the rear of the load. Then you can pre-tension the "line" This also means you can run the rig in reverse.

[thaimite]

The line pulling the load needs to be one continuous loop also connected to the rear of the load. Then you can pre-tension the "line" This also means you can run the rig in reverse.

That's the best idea yet if possible for your setup

[thaibeachlovers]

My suggestion is a flywheel to overcome varying speed. However, if it's a small motor, that may be difficult to add.

The worm gear mentioned before would probably work.

[1BADDAT]

I am sure that most of the ideas presented here could work with some fine tuning. The problem lies with that fact that If the OP had the ability to design and build most of them, then he probably already would have done it. Half of them would require parts to be custom made in a machine shop or have to order special parts from who knows where. In the US, I would be able to do this because I have full access to all the resources necessary. But I think it would be pretty hard in Thailand. In Engineering, the simplest solution that works is the best choice. We should keep that in mind when giving out advice.

[Bpraim1]

The line pulling the load needs to be one continuous loop also connected to the rear of the load. Then you can pre-tension the "line" This also means you can run the rig in reverse.

Hit the nail on the head. The best way to control speed.

[MaxwellsDemon]

Sorry folks, haven't had much time lately to reply and update but thanks to everyone who has!

I tried the incline and it works like a charm, but the incline option isn't very flexible nor portable.

I've gone back through all the posts and putting them all together would suggest lay the thin foam on the surface your wheels are running on.. Add the rolling resistance externally and you don't need to mess about with the wheels and or motor.

I'm definitely going to do this because some of the surfaces I'll use are definitely not smooth enough, so having a yoga mat or something to bring with me to turn any surface into a relatively flat and smooth surface will be good. I'll just have to make sure it doesn't introduce too much bumps, but I get the logic with this option and will try it when I have time.

The line pulling the load needs to be one continuous loop also connected to the rear of the load. Then you can pre-tension the "line" This also means you can run the rig in reverse.

Genius. Will do and report, though I don't see why it wouldn't work. Have to find a nice pulley system first.

[Semper]

Could you post a photo of this apparatus, when it's finished?

[MaxwellsDemon]

Ugh. So finally got a chance today to do some trials.

Bought a yoga mat from Tesco's, threw the still one-way system on it and BAM. Works like a charm! It's not very neat and it's a little fragile but it works!

So I decide to take it outside for a test run, pack my bags and then halfway there I realize I forgot to bring the spool of thread. Shit. So decided to try some nylon wire I bought at my local Tescos (too lazy to go home and get a bit of thread :/) remembering some comments about it here.

Everything was great, until I realized that the nylon string ended up reintroducing the problem on the yoga mat. It started bouncing again.

Suffice to say I was extremely frustrated, hot and sweaty at the nylon failure. Couldn't find any thread for sale, went home, sulked. Boo.

Next time.