What size servo/motor do I need?

Thread Starter

albsure

Joined Apr 1, 2012
14
Hello World. I'm mostly a software guy, enough to be dangerous with electronic hardware. My application is simple, just need to rotate a 5/8-inch steel shaft bidirectionally between 0 to 135 degrees. The shaft will be mounted in pillow block bearings with an estimated total load of about 15 pounds. How do I size my servo? I need to figure out how to calculate the torque ratings against this load. The speed can be slow, can allow several seconds for this rotation to occur, which is why I believe a simple servo should work better than a DC motor. Servo with metal gears is a given. TIA.
 

Bernard

Joined Aug 7, 2008
5,517
A possibility, standard servo with metal gears as illustrated by All Electronics Cat# DCS-116 with a Servo Saver, Cat# SRV-1, a spring loaded coupler to prevent shock loads on servo.
With programming might slow the motion to reduce over shoot oscillation ?
 
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Thread Starter

albsure

Joined Apr 1, 2012
14
A possibility, standard servo with metal gears as illustrated by All Electronics Cat# DCS-116 with a Servo Saver, Cat# SRV-1, a spring loaded coupler to prevent shock loads on servo.
With programming might slow the motion to reduce over shoot oscillation ?
@Bernard - thanks for the hint on the Servo Saver. It would be nice to have more info on this component, like measurement of the bore on side opposite the servo.
Also, with my board I have 12V relay controls and 5V pwm. Have any leads on servo's with 12V power and 5V signal? The ones I've found are industrial and quite expensive. Thanks again.
 

Thread Starter

albsure

Joined Apr 1, 2012
14
One method is to use the pulley and spring scale measurement in order to find the break-away torque, another is with a torque wrench if it can be applied to the end of the shaft.
Max.
To my shame, I avoided studying Physics. Fortunately my 17yr old son just schooled me on this. Thanks!!
 

Bernard

Joined Aug 7, 2008
5,517
Another way is to treat drive as a linear actuator with limit switches at 0 & 135 deg. with a high ratio gear reduced DC motor.
What is the control signal ?
 

Bernard

Joined Aug 7, 2008
5,517
Breakaway torque is about 7 oz. in. If shaft is about 10 ft. long, a little sagging might increase required starting torque ?
Where do you buy your parts? I've had success with small gear motors from " Surplus Center ",
Lincoln NE USA.
 
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Thread Starter

albsure

Joined Apr 1, 2012
14
Breakaway torque is about 7 oz. in. If shaft is about 10 ft. long, a little sagging might increase required starting torque ?
Where do you buy your parts? I've had success with small gear motors from " Surplus Center ",
Lincoln NE USA.
The pillow blocks should limit any sagging on the shaft.
I buy from anywhere... distributors, hobby shops, sparkfun, etc. Will check out Surplus Center as well, thanks.
 

Thread Starter

albsure

Joined Apr 1, 2012
14
Another way is to treat drive as a linear actuator with limit switches at 0 & 135 deg. with a high ratio gear reduced DC motor.
What is the control signal ?
Yah I've been considering gear motors also. Would really like to have something with a feedback signal because over-travel will definitely meet an immovable stop. I need a way to detect stall and stop driving the motor. Didn't mention that, before.
 

LesJones

Joined Jan 8, 2017
2,635
I can't see where the 7oz. in of torque that Bernard came from. The TS does not seem to have given any information on torque. I did think Bernard may have calculated it on the assumption that the 15 lb load was on the end of a cord wrapped round the 5/8" shaft but that works out as 5/16" x 15 lb = 4.69 lb. in = 75 oz. in.
Can you explain exactly what you mean by 15 lb load ? Does this device have to actually hit mechanical stops or would stopping when a microswitch was actuated meet your requirements ? Telling us what the purpose of this device would probably be helpful.

Les.
 

Bernard

Joined Aug 7, 2008
5,517
7 oz. in. from 5 lb. shaft rolling friction with 6 in. rod weighted with 2 nickels X 3.
Maybe we need to know more about the load, I had assumed that we were looking at starting friction of a long shaft.
 
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Janis59

Joined Aug 21, 2017
1,175
Generally, each stepper have specified the max torque. Simply find it in datasheet. Then apply a second Newtonian law F=ma, where load mass m, acceleration a, distance S and time t of chosen target is put in formula S=0.5at^2. The torque M to force F recalc ought b made by M=F*R where R is lever radius. So, the calculation contains no higher mathematics at all.
 

Bernard

Joined Aug 7, 2008
5,517
Assuming that the load is not out of balance by more than about 100 in. oz. then my choice of motors is Surplus Center Item 5-1796; 25 RPM, 12-24 VDC, 128 in.oz. , reversible, $ 11.95.
If the motor is shorted when not powered then there is less than a degree of overshoot.
A bidirectional circuit is shown by @vu2nan, page #12 , "12V DPDT relay wiring help".
What control signals are available?
 

Thread Starter

albsure

Joined Apr 1, 2012
14
Folks: Thanks for playing. You made me rethink a couple things, and now I'm pretty sure this needs to be a DC motor application rather than a servo, since I can't be sure about the angles. Also, there will be need to change the length of the shaft above the elbow which changes travel and angle, etc (ie. different configurations of a single design, not a dynamic reconfiguration).
By popular demand, I added a drawing and some more info.
Changing to DC motor should allow me to drive the load/wheel to the ground and then let the motor "float" the wheel on the ground until I need to bring it back to vertical/reset. A hall sensor on the shaft should let me detect vertical.
Controls are manual: Toggle switch or push buttons, just simple to tell it travel either one direction or the other (until it hits ground), or return to rest.
Application: agriculture and food plots. Sorry for being obscure.
2020-05-11 16_32_44-Window.jpg
 

Bernard

Joined Aug 7, 2008
5,517
Ouch! How long is the elbow ? If about 3 ft., then 5 lb. X 3 ft. would give your 15 ft. lb.
First step would be to add one or more torsion springs to counter balance most of the torque.
 

panic mode

Joined Oct 10, 2011
1,829
adding torsion spring can be used to compensate for gravity effects and determine minimum motor size. bit in order to size motor correctly you need to consider not just static but also dynamic load. motor requirements will be very different if this need to travel 180deg in 10minutes and or in 0.1second. so mass, inertia, acceleration are factors to consider.
 

LesJones

Joined Jan 8, 2017
2,635
Will sensing the positions for the 2 ground contact positions be good enough or does the wheel have to to apply a certain pressure against the ground ? Your idea of letting the motor freewheel towards the ground positions will not work if you use a motor with a high ratio gearbox between the motor and output shaft. (If it is a worm gear reduction it will definitely not work.)
On the control side will the arm always move to the rest position from either ground position or will it sometimes move ftom one ground position to the other ground position ? Giving a list of all the possible movements would be a help. It is difficult for us to design something when we are always just guessing what you want.

Les.
 

shortbus

Joined Sep 30, 2009
7,669
Being that this seems to be something for a farm tractor, and most of them have hydraulics available, and the movement is withing parameters that "could" work with a cylinder moving the arm it's self, as a bell crank, why not use a cylinder? Instead of a motor for the movement. Or if it has to be electric, use an electric linear actuator.
 
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