New to electronics. Need help with a student project involving a motor pulling an object with weight

Catarino Rodriguez

Joined Jan 6, 2017
2
My students want to create a pulley system that pulls a wooden dolly that will have someone standing on it. They want to use a motor to make the pulley pull the dolly with the student on it. The student weighs 60 pounds. The dolly just needs to move between 2 and 4 feet from the original location. Speed is not a factor. They are new to motors and after researching them, they see that it will have to connected to a power source.

What type of electrical or dc motors should they be looking at to do the required work?
Can electrical motors be connected to a wall socket? If so how would that be done?

I hope I provided enough information and have posted in the correct area. This area is new to me and my students. If more info is needed please let me know. Thanks for any help that can be provided.

#12

Joined Nov 30, 2010
18,217
I really have to discourage this. You have no business risking the safety of a 60 pound person if you don't even know that you plug things into a wall outlet with an electric cord.

Joined Jul 18, 2013
22,641
You would need to test the torque required, this can be done with a spring scale of sufficient capacity, A DC motor is probably the best, preferably with a gear box to obtain satisfactory torque.
Also you need to set up a standard emergency stop button to the circuit for operator use.
Max.

#12

Joined Nov 30, 2010
18,217
Right after you replace the person with 4 bowling balls, the formal way is to calculate the work required, then gear a motor down to a lower speed to inflict enough force to overcome the rolling resistance. How much rolling resistance? If you consider a boat, it's zero, or as one person told me, "You can tow a battleship with a sewing thread, but it will take all day." In the case of "no wheels" the force required would be the weight times the coefficient of friction (static) which can be a little more than 1. The answer must be between those two extremes, or zero to maybe 80 or 90 pounds of lateral force. Using wheels reduces the maximum force required quite a bit.

This suggests to me that a 60 pound person can provide the force, especially if pulleys are used to increase the mechanical advantage.
If you want to do it with a motor, I just gave you some numbers to work with.
You have to read the specs on the motor and calculate from there.

Catarino Rodriguez

Joined Jan 6, 2017
2
Thank you all for the information. I appreciate it. The students are going to use objects such as books and/or actual weights to use as weight until they get it to work correctly. I'll look at the specs as you have suggested. They had first wanted to use human force to make the pulley move the dolly but wanted to see if they could make the pulley mechanical through the use of motors. They are still in discussion mode and are planning out what they need to work on the project.

Joined Jul 18, 2013
22,641
Often for this DIY application roller blade wheels are used, fairly low in friction and cheap.
Or even sealed roller bearings.
Max.

drc_567

Joined Dec 29, 2008
1,088

The graph above illustrates the fundamental characteristics of a DC motor. At one end of the rpm scale, you have zero rotational speed, where the motor is stalled, and at the other, you have full rotational speed. At the zero rpm point, you have the maximum possible torque, but no rpm, so there is no movement. At the other rotational speed extreme you have the maximum rotational frequency, called 'no load speed' here, where no torque is produced. Consequently, in order to get the cart to move, it is necessary to select a motor operating point, on the diagonal line, that is between the two extreme rpm limits.
... Your pulley system must have a sufficient mechanical ratio such that it brings the required force needed to move the cart to within the torque capability of the motor. Note that torque is just a mathematical product equal to force*radius ,

Joined Jul 18, 2013
22,641
Most PM DC motors now have a fairly flat continuous torque curve with maximum at zero rpm and staying fairly flat or slow dropping until max rpm rating, where it drops off.
Torque is dependant on current.

Max.

drc_567

Joined Dec 29, 2008
1,088
^ ... Nevertheless, you must necessarily adjust the load force requirement so that it is within the range of the motor capability.
...It might be advisable, as a starting point, to select a motor and plot a few key points on its torque/rpm operating range.
... By doing this, you can get an estimate of what force ratio will be required from a pulley system.

Bernard

Joined Aug 7, 2008
5,774
Just to look at some gear head motors, try All Electronics, www.allelectronics.com. I used CAT # DCM-365
80 RPM, 6V, 110 mA; runs fine on 6V or 5 V AC wall adapter.
Sorry, that was the other motor, I used CAT # DCM-470. Window operators , also good choice. Say 50 RPM with a 2 in. drum, speed would be about 8 ft./ min.

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DNA Robotics

Joined Jun 13, 2014
599

1/2 in. Heavy Duty Variable Speed Reversible Drill
Only:$29.99 http://www.harborfreight.com/12-in-heavy-duty-variable-speed-reversible-drill-69452.html Already gear reduced and plenty strong. Where that handle is on the side, there is a threaded bolt hole and one on the other side. Great for mounting it on a stand. Chuck a shaft or spool in it to wind up your rope. How a Block and Tackle Works | HowStuffWorks science.howstuffworks.com/transport/engines-equipment/pulley.htm Block and tackle https://en.wikipedia.org/wiki/Block_and_tackle You can get a block & tackle like this on EBay for$16.00.
You can demonstrate the principals by using one pulley to start and adding them one or 2 at a time to see the difference.
4000LB 2 Ton 65FT Poly Rope Hoist Pulley Block And Tackle Rope 7:1 Lifting

ClassOfZero

Joined Dec 28, 2016
114
Low speed torque of electric drills aint all that great, I'd question it's ability to pull the skin of custard at low speed, even more so if it doesn't have a hi or low gear option.

Joined Jul 18, 2013
22,641
If this were a permanent application, it would require a soft start feature, the same way T.M.'s do to prevent instant full or high speed start from zero.
Max.

Tonyr1084

Joined Sep 24, 2015
5,810
I used to use a cordless drill (Dewalt 18v) on low RPM to lift a truck shell off the back of my pickup. Using that along with a worm drive gear and pulley to pull a block and tackle to lift the shell off the bed of the truck. It was slow but worked nicely. I no longer have the truck and shell, hence I've repurposed the worm drive (also known as a screw drive) for other stuff. I got the worm drive out of an old small roto-tiller. A Honda I believe.

When I started reading this thread my immediate concern was safety. Messing with electricity straight out of the wall by a novice and exposing kids to it made my hair stand on end. If anything, I'd go with battery power. If you need to keep something plugged in - use a battery charger to maintain the battery voltage. Let the battery handle the high current during operation and the charger to restore the battery during rest.

Teaching children mechanical engineering (that's what you're doing) is great. When I was as young as 7 years old I was messing with all kinds of stuff - to my fathers chagrin at times. And I received my unfair share of electrical shocks too.

Teach them safety first and they can avoid learning things the hard way. 120 VAC is not the kindest to muscles. Especially across the heart muscle if ever accidentally exposed. So my vote goes for using a battery powered motor. You can grab a starter motor from a junk yard. One that is of a gear reduction design. Small, high spinning, gear reduction, lots of torque. Use a small enough drive shaft to wrap a rope around and you might not even have to use a block and tackle setup.

There you have it. A car battery and a car starter motor. Low voltage but very high torque. Just make sure the kids don't get body parts trapped in moving parts. Please.

#12

Bernard

Joined Aug 7, 2008
5,774
Re post # 14. A starter motor is not kids play, way too much torque. A seismograph recording truck carried a large reel, about 48 in. wide 28 in. end plates friction driven by a Ford starter motor could pick up 1/2 in. cable
@ 30 MPH. 1950's. I lost several gloves, but no fingers.

Joined Jul 18, 2013
22,641
There are starter motors and ....
The older type was series wound motor which has incredible torque at zero rpm but with no load, can run away, this is why they should never be ran on the bench or with no load.
Many of the current types are shunt, P.M. motors and can be controlled like any other shunt DC motor.
Some of the AC types used on snow blowers mowers etc, are also series (Universal) motors.
Max.

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Tonyr1084

Joined Sep 24, 2015
5,810
Re post # 14. A starter motor is not kids play, way too much torque. A seismograph recording truck carried a large reel, about 48 in. wide 28 in. end plates friction driven by a Ford starter motor could pick up 1/2 in. cable
@ 30 MPH. 1950's. I lost several gloves, but no fingers.
Of course safety is at issue. Machinery and potentially dangerous electricity around children was my first concern. My vision was with something out of a four cylinder Dodge. Since I haven't any experience with this I was thinking outside the box. A lawn mower starter motor that runs on 12 volts or on 120 volts AC like Max suggested (he said snow blower - same thing) sounds plausible too.

Just remember, "SAFETY" is rule #1. Safety from moving parts AND from potentially dangerous electricity. (you can include chemicals on that list if you opt for a car battery as well)

Tonyr1084

Joined Sep 24, 2015
5,810
Just glanced at an old garage door opener. The motor is 130 VDC. It's VERY geared down, so that'll likely yank the teeth out of a student. Not that you want to, but the speed of which the garage door opens or closes, it's not going to be a sudden whiplashing jerk. Slow and strong. All you'll need to do is rectify the AC to DC and then drive the motor in one direction or the other. Use a double pole double throw with center off to accomplish this.

OR you can use a four way wall switch to control direction and a regular switch to turn it on and off. Just put this stuff into a junction box to keep prying fingers out of harms way (from electricity).

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Joined Jul 18, 2013
22,641
Just glanced at an old garage door opener. The motor is 130 VDC.
That Must be a very old one, I don't recall ever seeing a DC motor on a garage door opener, for some time now they have been mainly AC PSC motors.
Usually worm drive which is inherently low gearing.
Max.

Tonyr1084

Joined Sep 24, 2015
5,810
Max: I understand the skepticism. I find there are things I've never experienced before, but there's always a first time. Tread mill motors are often 130 VDC. Knowing my Genie GDO was also a DC motor I assumed it was 130 VDC. It's not. It's actually listed as 140 VDC. I guess DC motors are more easily reversed.

And yes, this is worm driven as well.

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