19 replies to this topic

[Arconious]

A couple items that I'm planning on using on a project of mine. Does anybody know where I can get them?

I'm looking for a simple battery operated motor, Preferably a rotary action motor. With a decent deal of power behind it.

And a old style film canister. Or something similar.

I'd like something cheap but as long as it gets the job done, I could deal with price.

[Blue]

PM me if you're interested.

[wespelarno]

A couple items that I'm planning on using on a project of mine. Does anybody know where I can get them?

I'm looking for a simple battery operated motor, Preferably a rotary action motor. With a decent deal of power behind it.

And a old style film canister. Or something similar.

I'd like something cheap but as long as it gets the job done, I could deal with price.

Decent amount of power depens on application. Explain what you intend on doing with the the motor and I can suggest a motor to suit your needs.

If you use a DC power source (batteries), there are three basic motor types

Brushed
Brushless
Coreless

In this instance you want a brushed motor as they are cheapest and will provide suitable power. Brushless are for really high rpm high power applictions (i have one that turns 42000rpm at 7.2V) and coreless won't have sufficient torque.

Once you are into brushed motors, there are a whole variety of types. I would recommend a mabuchi or silver can motor (two different names for the same thing). They are again simplest and will be least hassel in a nerf gun.

The brushes aren't replacable, so when the motor burns out it is dead. You can buy motors with replacable brushes, but again for this application it is unnesescary. And in a nerf application expect quite a few hundred hours of use before you will want to replace it.

Mabucchi motors come in a variety of sizes. Here are some rough guidelines:
The number of a motor indicates its size. 540 is smaller than 550 but bigger than 380. The bigger the motor, generally the more torque it will have.

The rpm is controlled by the number of turns the motor has. This is how many loops of wire there are around the commutator. More loops=more resistance=lower rpm. However, lower rpm gives longer motor life. a 27 turn 540 motor will turn 20,000rpm unloaded. a 8 turn 540 motor will turn 50,000rpm unloaded (both at 7.2V)

There is a sacrifice in having higher rpm. Because there is less mass rotating around the commutator, the motor will have less torque. To compensate for this, motors come in a variety of winds. You can basically ignore this as it doesn't make a huge amount of difference.

If you need more power, you can increase the voltage to the motors up to about 15V before they get unhappy. They are designed to be run at 7.2 but they are robust enough to be run at higher power. Again, this will reduce motor life so "ye be warned"

Goodluck, to get a solid understanding of electric motors takes quite a bit of effort

Edited by wespelarno, 21 January 2009 - 03:53 AM.

[Arconious]

Mind if I PM you asking for a bit of advice? I'm looking for something with a very specific purpose.

[Wes7143]

Xmod RC car motors are really nice high-torque motors. You can buy just the motor for around $15 at RadioShack.

Just out of curiosity, may I ask what you plan to do with the motor?

[CaptainSlug]

I'm looking for something with a very specific purpose.

And yet you've provided no specifics.
Even an RPM range would be vital for someone willing to provide advice.
Or better still what voltage and capacity of battery pack you want to power it from, what kind of load it needs to be able to apply, any dimensional considerations, and so forth.

I'm inclined to just tell you to go to a library.

Edited by CaptainSlug, 21 January 2009 - 07:47 PM.

[wespelarno]

Mind if I PM you asking for a bit of advice? I'm looking for something with a very specific purpose.

Sure. Give me a buzz anytime. I'll see what I can do to help. If it is a very specific application I will need details about what you intend to use it for, size restrictions (if any) and any other stuff you consider relevant.

[Arconious]

Well, I'm looking for something that could turn something that weighs almost ten pounds at around 5-10 RPM.

High power, low RPM.

I'm thinking anything below 6-8 inches wide would be usable.

I don't have a battery pack, I was planning on picking one up when I got the motor. So that isn't a limiting factor yet.

[adama]

That kind of application needs gear reduction. This has suddenly become a very expensive project for you. With that said I would check out andymark.biz or the Banebots website. They make gearboxes for hobby robotics applications and would most likely have a product to suite your yet to be explained purposes.

[Draconis]

You want something like one of the cheap $15 cordless drills from Harbor Freight. Then just adjust the voltage to the motor. You might be able to use something like a cordless screwdriver too.

[CaptainSlug]

High power, low RPM.

Then you need a gear motor of some kind. 10lb is a fair amount. You should look at windshield wiper motors, motors used for power wheels, or gearhead motors.
Cordless power drills typically max out at 20lb/ft of torque and might be suitable.

[Arconious]

I didn't expect it to be cheap when I made the design. So the cost won't really bother me.

Thanks for the ideas. I'll look into them.

[wespelarno]

I didn't expect it to be cheap when I made the design. So the cost won't really bother me.

Thanks for the ideas. I'll look into them.

I take it this will be a stationary project, ie not one you intend on moving with? Cause a motor and battery set up this large, plus 10 pounds of weight will be really heavy.

If so, I would look into a hobby size lead acid battery. Cheap, easy to charge and maintain a very steady 12 volts, but can not be used for moving projects.

Also, the 10 pounds you are spinning, how big a radius are you spinning it on, or will it be centered around the motor armature?

Motors, you will definately require a gearbox. With absolutely huge reduction. Electric motors excel at high RPM and aren't so great at massive torque. I would be surprised if you could obtain a motor than span much less than 5,000rpm at full power so you are looking at a lot or reduction. I would be looking into cordless drill motors (900 size from memory). Even they are likely to struggle at this effort. Maybe a lathe motor? Bear in mind most lathe motors are 120V. I

would seriously suggest going to a store that specialises in power tools and talk to them. They will quite likely be able to order replacement parts for the tools they stock so you should be able to pick up motors and gearbox (for a price). If you are lucky they might have some trashed tools out back you can stip for parts. Sorry to say I don't have much experience with motors laying out this type of power or of this size.

[Arconious]

Well, I'm planning on moving with it. I'm quite strong, so it shouldn't be a problem. I can lift a Oak Log pretty easily so, it shouldn't be too heavy for me.

What do you mean by that? I can detect three or four meanings out of that without actually knowing which one you meant.

I'm not posting what this is for because I want to get it working first. If I tell you and horribly fail, I'll feel bad about it. But if I don't tell you and fail, all I have to say is that I failed. Easier IMO. Besides, idea threads stink, and I don't want to say I can do something I haven't done yet.

[wespelarno]

If you are planning on moving with it you can't use a lead acid battry. The way most are set up, if they are inverted or tipped an an angle apart from vertical all the chemicals will mix and at best kill the battery, at worst melt down in your hands. If it is a moving project you will need to look in alternate battery sources which will be expensive in comparison. Because of the mass you are moving it will need a battry with a high MaH rating. I would recommend numerous LiPo batteries soldered in parallel so that six 5,000MaH batteries will act as one 30,000MaH battery. The other advantage is that LiPo batteries are light compared to lead acid batteries. You could use NiMH cells which will be cheaper than LiPo cells but weigh more and have a lower MaH rating so you will need more of them. You will also need chargers which are more expensive than lead acid chargers due to LiPo cells being more sensitive. So, Overall, LiPo batteries are lighter, higher MaH and more expensive. NiMH, heavier, lower MaH but much cheaper. I can give more details or cells I'd recommend if you want. If you can afford it, LiPo's would be better.

Other considerations:
You will also need to rig up something to hold all the cells in place.

Get your hands on some Deans ultra plugs. They are connectors that have very low resistance (they actually have less resistance thant he equivalent length of 12 gauge copper wire) so won't melt under the load they will be under. Normal tamiya connectors won't last long and would melt. I assume you know how to sodler?

I'm not doubting you can lift an oak log, but can you run round and duck behind cover carrying one? This much weight will be cumbersome to manouver with. I have no idea how fast you intend on moving with it, but this project is likely to weight as much as a pallet of bricks.

My question regarding the 10 pounds being centered or not. If it is attatched centrally to the motor's rotor, it won't need balancing and it will only be 10 pounds for the motor to spin. If it is attatched to an arm that spins around the rotor, it will need an equal weight on the opossite side to balance it (doubling the weight on the motor) and due mechanical advantage (the weight turns a larger circle when on an arm, and therefore moves faster) will require a much more powerful motor. I've got formulas to show the proportional increase in force for a greater arm length which I can dig out if you need. From memory, increase arm length by a factor x, the force required increases by a factor of x^2 (x squared)

Another thought towards motors is a gearbox that allows you to connect to motors to a central spur gear, giving you two motors to power it with. This will allow you to buy two smaller and more likely cheap motors as opposed to one huge and likely expensive motor.

[Arconious]

Considering the only thing I have to put my money on at the moment is video games and nerf guns, I'll probably be able to afford the better batteries. I'd prefer something of more quality anyway.

I could probably get something together.

Alright, I assume I can get those at a hardware store. And yes I can solder. I'm not great ATM, but I know people who are alot better, so if it gets over my head, I can go there.

I'm not hugely fast anyway. So I can deal with it being heavy.

The ten pounds will likely be connected by a gear system.

If I need one anyway, that would probably be good. Allow me to save some cost here.

[wespelarno]

Considering the only thing I have to put my money on at the moment is video games and nerf guns, I'll probably be able to afford the better batteries. I'd prefer something of more quality anyway.

I could probably get something together.

Alright, I assume I can get those at a hardware store. And yes I can solder. I'm not great ATM, but I know people who are alot better, so if it gets over my head, I can go there.

I'm not hugely fast anyway. So I can deal with it being heavy.

The ten pounds will likely be connected by a gear system.

If I need one anyway, that would probably be good. Allow me to save some cost here.

Ok. Lipos are about $100 each for a 5000MaH battery that runs at 7.2V. So you are maybe looking at $300, $400 of batteries? NiMh would be about half that. Another thought I had. LiPo batteries tend to go up like a quarter stick if abused, overheated, punctured etc. In an application where they could concievably be landed on and punctured if you fall, or overheat if used on a hot day and there isn't much battery ventilation, NiMh batteries might be preferable. Unless you design a battery holder that offers puncture protection and very good ventilation.

Now. Onto cooling. Those motors will generate a lot of heat during continuos usage, maybe more so than normal because they will be under a lot of stress. So you will need to keep them cool. I would recommend CPU fans. If you can, have the motor situated in a a peice of PVC pipe that leaves about 4cm on either side of the motor can. Then attach a CPU fan at the end of the pipe. so it creates a wind tunnel around the motor. A design like this would keep the motor nice and cool. You will need to use a motor plate (That screws directly to the front of the motor) to hold the motor in place as opposed to a metal bracket. You may need something similair for the batteries. Look into heat sinks for the motor as well. That will be a cheap way to aid cooling. I've had motors heat up so much the solder holding the wires together melted. Plus the heat damages motors and batteries and reduces their useful like span. I would also drill ventilation holes into any case you make to allow more cool air into the components.

If you want an adjustable rate of fire, a resistor could be soldered in series with the firing mech to allow you to conserve ammo if needed.

Deans plugs are available from hobby stores. Just ask for deans ultra plugs. The store owners will know what you mean. I would use them for all battery connections. The motors should be hard wired (directly soldered to the wires) to give less resistance and one less point for stuff to go wrong.

[wespelarno]

And what if then, you learn that you could have done all the same things for maybe $45, by using sealed lead-acid (SLA) batteries? They don't have any of the problems claimed a few posts back, often cost less than ten percent as much, and the gear for charging them is cheaper, too.

The guy who runs my local hobby store was the one who explained the stated issues with lead acid batteries to me. And I have seen a lead acid battery die from being inverted whilst at the local RC track. That said, if Lead acid batteries are available that do not have the issues i mentionted then that is a good way to go. When I asked the hobby store owner he simply said they didn't exist. Otherwise, I stand corrected.

[Nobber]

You know, I think you could get all the stuff in a hobby store.

[Arconious]

@Just Some Bob- I wasn't planning on being stupid about it. I'm taking all the advice given, but I was planning on testing everything before I went out and bought the expensive batteries.

This is a concept, and since I don't think its been done on a nerf gun before, I expect parts of it to be buggy and not work the way I want to as soon as I put it together.

@wespelarno- I appriciate all of your advice, and I'll take the suggestions you give me into account. Even so, I'm probably going to research some of this just to get a varied perspective on the problem. Its possible that the man in the hobby shop lied to you in order to get you to buy more expensive batteries.

@Nobber- Probably. I'm getting some advice on what to buy when I go to get everything now.