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I would like a high torque motor (37 oz-in @ 5760 rpm) for souping up a Scorbot 3 I bought. I really need it to have an encoder to count the number of revolutions and to allow high start-up torque. So far, I'm having difficulty finding a suitable motor.

The closest I've found are:

  1. Revolver S Stubby (still not ready for purchase)
  2. Team Novak Ballistic 25.5T

I've found other RC car motors, but they are usually too big.

Some alternatives I thought about are:

  1. adding hall sensors to an existing motor - how hard is this?
  2. rewinding a motor with more turns to increase torque (decrease Kv)

Does anybody know of any motors that fit these requirements or modifications I can make to existing ones?

Update: I had almost given up hope, until someone at Homebrew Robotics suggested using the Maxon motor finder.

If you just type in my given torque and speed, it returns 3 motors, but they're all over powered because the search interprets your specs as a continuous operating point, whereas my robot will only need that much power 20% of the time, and maybe for 1 second max.

If I type in 12V, 5000rpm, and 15 oz-in, then it returns 2 brushless motors, of which, the Motor EC 45 is the best fit, which has this operating curve:

http://msp.maxonmotor.com/camosHtml/i?SIG=fb9a5d91198caf381122a3d6eab8b1bda3877f30_fa_1e0.png

However, I don't want to pay what Maxon is charging, so instead, I've contacted the guy who makes the yet to be released Revolver Stubby and he has kindly offered to build a custom high torque, low RPM motor for me.

Can anyone comment on why high torque, low RPM motors like the one I want seem so rare? Is due to lack of applications (robotics) or is there some intrinsic difficulty in making them?

Mark Booth
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Yale Zhang
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  • Welcome to Robotics.SE - unfortunately, questions such as this are referred to as "shopping list" questions, and have no "right" answer. – Andrew Jan 06 '13 at 08:46
  • But it will only have a few answers if any at all! I have searched long and hard for such a motor and haven't found any yet. I really think this is a useful, legitimate question. – Yale Zhang Jan 06 '13 at 09:46
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    The number of possible answers is (unfortunately) not relevant because your question is a subjective one. Imagine asking "What sensored motors operate on at least 0.1 watts?" -- there are hundreds if not thousands of motors that fit this description, so choosing one of them to be correct would be an arbitrary choice. For that reason, this form of question is wrong for this site, no matter how specific the criteria.

    Instead, try your question in one of the chat rooms!

     – Ian Jan 06 '13 at 18:40
  • For people new to robotics who are confused about why stack all exchange sites try to avoid shopping questions please see the blog post Q&A is Hard, Let’s Go Shopping!. – Mark Booth Jan 07 '13 at 10:57
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    I'm not sure I agree that this is a "problem" shopping question. It's only incidentally shopping-related. I think it's actually, "I'm trying to do this thing, and I'm open to any solution: I've considered both building/modifying existing motors, and buying one that could do it, but haven't found anything that works for either." That's not really the same as, "there are tons of things that do this thing, which is best for me". – Jaydles Jan 09 '13 at 16:35
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    Please see How do I ask a question that may require recommending a product. This question doesn't really fall into the problematic "shopping questions" often discouraged on these sites. The author is asking about a well-researched problem which may need a specific product to fulfill that need. That's not the same issue as "what's the best smart phone" -type question for which that blog post was written. Thank you for the discussion; I am going to re-open this, but feel free to discuss in meta if need be. – Robert Cartaino Jan 09 '13 at 16:45
  • I could see replacing this question with 2 much better questions: (1) How do I add hall sensors to an existing motor? and (2) How do I re-wind motor wiring with more turns to increase torque? Those are both questions that would be valuable individually. – Ian Jan 10 '13 at 17:53
  • @Ian, while these would indeed be good questions in their own right, there is nothing inherently wrong with asking a question which has several distinct parts which all contribute to the whole. – Mark Booth Jan 21 '13 at 12:21
  • @RobertCartaino - I was going to wait until the question was improved before re-opening, but thanks for the link, it does help us understand the difference between a question to which a product recommendation might be a solution from pure shopping questions. – Mark Booth Jan 21 '13 at 13:25

1 Answers1

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One possible solution to your torque problem is to use mechanical advantage to trade speed for torque.

If you replace a 1:2 gear/drive belt combination with a 1:1 gear/drive belt combination then you will get twice the torque at half the speed. Swap the gears and you can use the same drive belt to get a 2:1 ratio instead and get even more torque for even less speed.

Of course you will have to re-tune your motors, the PID control parameters will be completely different with the new gearing.

Mark Booth
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