Help: Motor and Worm Gearbox Torque 1

[Sirius2]

Hello. I hope somebody will be able to help.

I have designed some equipment to assemble some large parts. It involves a stand and a fixture attached to it via a worm gear box.

The fixture and part needs to tip over at 90 degrees, from vertical to horizontal, and back.

After design and build, the specification has changed. I am not sure if the motor is going to be strong enough.


The total weight being lifted is 120kg.

The distance of mass from the pivot point is 280mm.

The current geared motor is 0.127 Nm. It is planetary geared to 100:1. Roughly I think this gives 12 Newton Meters.

The motor output Rpm is 60.

I have hooked this motor up to a 30:1 NMRV worm gear box, because 60rpm is too fast. I ideally wanted to rotate the fixture 90 degrees in less than 10 seconds, but no more than 20.



I have read that the calc for Final Torque is: Initial Motor Torque x Gear Ratio.

I have therefore taken the original motor torque and multiplied it by 130:1 total gearing. 0.127 x 130 = 16.5 Nm.


So far so good?.....


My weight is 120kg at 280mm from pivot. To get this back into meters, I have divided One Meter by 280mm, which gives 3.55. (Ie, 280mm fits 3.55 times into a meter).

120kg divided by 3.55 = approx 34kg at 1 meter. For certainty I am calling it 35kg.



So I have a mass of 35Kg at 1 meter from the pivot, which is then being worked back through a 30:1 worm gear box, then back to the 100:1 motor gearbox.

Is any of this right?



Can anybody help me understand what I now need to do with this 35kg, to know what the output torque of the motor needs to be, prior to the 30:1 gearbox?



Ie, taking the motor as it is now, at 12 Nm, or 16Nm through the second gearing, would this be enough to lift the weight?


Sorry if this is hard to understand, or something that is normally very simple for you guys. (Automation and motors etc are not usually my kind of work).

Many thanks

Sirius.

 

[Sirius2]

Looking through this again, am I right in the following?.....

I have used an online convertor to convert 35kg force per meter into Newton Meters.

35kg is allegedly 343 Newton Meters. To get the motor torque, would I then divide this by the 30:1 gearing on the worm box?

343 Newtons per meter divided by 30:1 gearbox - 343/30 = 11.43 Nm.

My original Motor is capable of 12 Nm.....

Am I doing this right?

Many thanks.

 

[BrianPetersen]

Required output torque is simply 120 kg x 9.81 N/kg (gravitational constant - I presume that your mechanism is located somewhere near the surface of the earth) x 0.28 m (length of moment arm) = 330 N.m

This is the output torque of the 30:1 gearbox. Without consideration of the frictional losses (which will be significant, in a worm-drive gearbox) this will require 330 / 30 = 11 N.m input torque.

That is also the required output torque of the first gearbox (without considering friction). Since this one is simply 100:1, the required input torque of that gearbox, which is the output torque of the motor (not the gear-motor ... the MOTOR), is 0.11 N.m. If you bought an integral gear-motor (motor + gearbox combined) and you have the output torque and RPM of that gear-motor then the internal contents of that gear-motor are irrelevant to you. The gear-motor has to output 11 N.m (plus something for frictional losses in the subsequent mechanism).

Frictional losses in that worm-drive gearbox are gonna getcha, though. The efficiency of a worm-gear reducer varies depending on the design of the internal gears ... but it's never very good.

 

[waross]

The type of motor may be important.
Sticktion may be more important than friction.
"Anecdote on"
Many years ago a particle accelerator called Triumf was constructed. (Tri Universities, Meson Facility)
Part of this was a large chamber operating under a vacuum.
A number of motor driven screw jacks were used to lift the top of the chamber when access was required to the inner part of the vacuum chamber.
Everything was calculated based on the friction of the gear reducers and the screw jacks.
The top of the chamber was lifted as expected.
Then there was an occasion to stop the lift partway and then continue.
Sticktion.
The motors did not have enough torque to overcome the sticktion and start the screw jacks when they were loaded.
After you have determined the friction that you must overcome, (The gearbox maker may list the losses of the gearbox) you may want to consider sticktion also.
Most motors will develop greater than nominal torque for a short period but we need some motor specs to advise you on the probable available torque.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

Thanks Brian and Waross.

Brian, you make it sound so easy!

It looks like I have wandered around the houses to work things out backwards, but I had somehow come up with approximately the right answer.

Just out of curiosity, would you agree that the methodology I used is (kind of) right, even though this is not the proper way to do it?

Of course, yours is much more straight forward, thank you very much!

Waross, I have never even heard of sticktion before. I will have a look into that. I need to try and find some kind of figure to take into account the loss of efficiency for a 30:1 reducer too.....as things seem to be getting quite to the limit of the existing motor.

The motor is an integrated gear-motor. A "99.5:1 planetary/epicyclic, three stage reduction, all-steel gears with large-size carrier pins for high torque handling"....

Nominal voltage: 12V DC
Operating voltage: 3-18V
No-load Motor Speed (minimum.): 6000rpm
No-load Current (w/gearbox): 0.6A
No-load Speed after gearbox: 58rpm
Max. Efficiency Speed after gearbox: 48rpm
Max. Efficiency Current: 2.3A
Max. Efficiency Torque: 2.1Nm (21.4kg-cm)
Stall Current: 11.4A
Stall Torque, motor only: 1300g-cm
Stall Torque after gearbox: 11.9Nm (121.3kg-cm)

The worm box is something a bit like this, but without the motor mounting casting:

http://www.power-reaction.co.uk/Cat...ollow-Bore-R30-1-63B14-MOT-NMRV40-R30-1-63B14

Changing the voltage will cause issues with the control box.

To try and salvage this job, if the motor is a bit too weak after the efficiency loss, I have searched for a 12v motor with a higher torque and found this

http://uk.rs-online.com/web/p/dc-geared-motors/7736771/

Output Speed 70 rpm
Supply Voltage 12 V dc
Maximum Output Torque 3 Nm, 25 Nm
DC Motor Type Brushed
Shaft Diameter 9mm
Gearhead Type Worm
Length 178mm
Width 60mm
Depth 100.6mm
Dimensions 178 x 60 x 100.6 mm
Current Rating 6 A, 34 A
Core Construction Iron Core

If this gives me 25Nm, and the other was 12Nm, I gather that this would more than cover for the efficiency loss.... so I suspect it is worth spending the money to make sure it works.

There is another motor for half the price which has an output torque of 14.7 Nm. This would be easier to fit because it is the same shape and diameter as the existing motor. However, is the extra 2 or 3 Nm enough for the efficiency loss?


Thanks again.

 

[waross]

You may be undersized by a factor of around 5:1
You should be using the maximum efficiency torque.
Stall torque is the maximum momentary torque.
Designing for continuous running at stall torque may result in early motor failure.
Friction is usually sliding friction.
Sticktion is static friction. The force required to start an object sliding.
From the specs of the second motor:
Continuous current = 6 amps.
Stall current = 34 Amps
I[sup]2[/sup]R losses at stall current = (34/6)[sup]2[/sup] = 32 or 3200%
The good news is that with a ratio of stall torque to rated torque of over 5:1 you probably don't need to worry about sticktion.
But friction is not my field. Hopefully a mechanical expert will post a typical ratio of friction to static friction for a worm drive.
Running some gearboxes at maximum torque may result in overheating and/or early failure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

Thanks for the extra insight Waross.

I was thinking of ordering the second motor, from R.S. Components, but now, I don't know any more.

The calculation for the needed torque is now established to be about 11 Nm. The motor allegedly outputs 25 Nm, more than double.

I do not know whether this is the "stall torque" though. If it was, could it potentially handle around 20Nm without grumbling too much?

(I had hoped this 'doubling up' would be enough, including any loss in the worm gear reducer). The maximum strain would be the initial movement and upwards to -45 degrees or so, - then the weight of the fixture and the part would help do the work as it tips over the pivot point. Likewise for the return journey.

The fixture is not being constantly motored to 90 degrees and back. It will be loaded with a part, powered backwards 90 degrees, some work will be carried out, then it will be lowered. Then another part will be loaded, that part will be worked upon, then it will be tilted back, then worked upon again, then lowered again for unloading and then loading a new part.

The fixture caters for several similar parts, not all of them are needing to be tilted back, so at worst case scenario it will be a "day shift" of usage on that particular part number, with the operator doing maybe 4 or 5 parts per hour, if that helps. I am hoping, therefore, that it is less intense than a normal motor that would be whizzing around all day long...

I am struggling to find a 12 volt motor with a higher torque, particularly one which is easily available and not many £ hundreds. I might have to investigate adding some counterbalance weights to lighten the strain, but it is not ideal and won't look too professional.

Are you thinking that the DOGA brand motor (rated at 25Nm) will be too weak, or do you think it would be able to cope okay?

I may be able to get a different worm gear box, the same sort, but a ratio of 40:1 or maybe 50:1, depending on the extra time it would take to rotate it 90 degrees.....but I am not sure if any extra torque gained this way would make that much difference.

 

[waross]

RATED TORQUE ON THAT MOTOR SEEMS TO BE 3 Nm
STALL TORQUE ON TAT MOTOR IS 25 Nm
Yes the motor will develop peak torque of 25 Nm.
However the motor will be overloaded at anything past 3 Nm.
Motor heating will be roughly (Actual torque / Rated torque)[sup]2[/sup]
(11 Nm / 3 Nm) 13.4 or 1344% Even for a short period the motor may run a little warm at over 1300% of rated losses.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

I'm sorry for being a bit slow on the uptake of this Waross.

As you may have guessed, I know next to nothing about motors, or torques, or amps and volts. My usual work is press tooling, jigs and fixtures to inspect components. It has been 20 years since I did any serious maths or science at college, because I have never needed to use it in all these years.

I am having to try and pick it up as I go along, flying by the seat of my pants (as usual).

I read the other week, when looking around for info on how to work out torques and gears (which may not have been the right thing) that the "Final torque" is the initial motor torque prior to gearing, times by gear ratio.

On this basis, I have been looking a the 3 Nm torque and assuming this was the motor torque before it went through the gears on the motor. After the gears, I have been assuming the motor outputs at 25 Nm. Maybe I am going completely wrong here. I hope it is not the case that the geared motor runs "normally" at just 3 Nm, but grinds to a halt and burns out at 25Nm. I would therefore be running it somewhere around half the burnout rate.

When I tap the search results into the RS catalogue selection, it states the motor has an output torque of 25Nm. The stall torque of the DOGA motor could perhaps be that, or maybe more, I am not sure. I am not sure what I am missing here. I had hoped this was the torque it could normally handle okay.

When you say that the motor will be overloaded at 3 Nm, this sounds disastrous....... 3Nm, and I need at least 11. Or are you saying it would lift it, but it would be getting red hot very quickly and would soon burn out?

Furthermore, I am also reading the specification as though the final output of the gear motor is 70 RPM. Looking at the catalogue PDF, now I am not even sure of that! It says on the RS configurator that the output speed is 70 RPM, so I am taking their word for that too, like the output torque.

I think I need the motor to give me somewhere between 58 and 70 RPM when working the fixture. (I need roughly 2 RPM by the time it is reduced 30:1 via the secondary worm gearbox it plugs into)

If the initial motor speed is 70 RPM, but this thing actually crawls around at say 10 RPM when loaded, by the time it passes through the 30:1 reduction, it would take an eternity to rotate the fixture 90 degrees.

https://www.rapidonline.com/pdf/560993T_v1.pdf

 

[Sirius2]

This is another one, twice as much money, but 12volts and it seems to be 40 Nm instead of 25 Nm.

http://uk.rs-online.com/web/p/dc-geared-motors/7736793/

Output Speed 100 rpm
Supply Voltage 12 V dc
Maximum Output Torque 4 Nm, 40 Nm
DC Motor Type Brushed
Shaft Diameter 12mm
Gearhead Type Worm
Length 206.5mm
Width 60mm
Depth 122.7mm
Dimensions 206.5 x 60 x 122.7 mm
Current Rating 6 A, 60 A
Core Construction Iron Core

I could probably live with 100 RPM. It is a bit brisk, but it is better that than too slow.

Is this any better?

 

[waross]

Maximum Output Torque 4 Nm, 40 Nm
40 is stall torque.
4 is rated torque.
(11/4)[sup]2[/sup] = 7.56 only overloaded by a factor of 756%.
Getting closer.
Maximum Output Torque 4 Nm, 40 Nm
Rated torque is 4 Nm


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

What happens in-between the 4Nm and 40Nm? That sounds rather a large gap.

If I needed 10Nm to lift 120kg - and it can handle 40Nm without grinding to a halt, would I not just be running it at 25% more strain than it ought to run? Would this be a problem?

I am sorry, I am not quite understanding whether I am miles out.

Being overloaded by 756% sounds like this motor I am aiming for is like me putting a Lego motor into a real world motorbike, and is thus a joke.

I really have no idea what I am doing with this any more.

I thought I was along the right tracks, but I may be about to spend a lot of money and make things worse.

I have been told that I cannot use a motor and gearbox combination (such as Bonfiglioni or Motovario) because the voltage is 230, and that we would not be able to reverse the motor back and forth at 90 degrees. Also, there is a 12volt linear actuator that lifts the fixture stand to different working heights. I was told this was also a problem if adding such a gear motor at that voltage, and that it was a lot more unsafe compared to the low voltage motors (in terms of operator electrocution).

 

[waross]

You are confusing working torque with stall torque. The motor will develop stall torque when it is stalled. That is at zero RPM.
Motors develop stall torque when starting. As the speed increases, the torque drops.
At stall the motor is badly overloaded.
Your current rating is 6 A, 60 Amp.
6 Amps is the rated maximum safe continuous Amps.
60 Amps is the momentary starting current.
Motor heating is mostly I[sup]2[/sup]R
If you double the torque demand, you double the current. That means 4 time the heating.
When you see 4 Nm: 40 Nm, use the 4 Nm for design, not the 40 Nm

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

Many thanks for your patience on this Waross.

I can appreciate it must be frustrating for you when I don't know anything about amps, or I2R, or how this has been effecting the torque issue.

Well, it looks like I am up the creek without a paddle on this one. I really don't know what to do next.

All the controls and electrical equipment has been made to suit and power actuators and motors at 12 Volts (for the original requirement of 40kg, not 120kg). This was approx £1000, including safety stops, foot pedal controls, limit switches, relays, cables, electrical diagram supplied and so on. A linear actuator is yet to be bought.

When we did an initial test of this, clamping some 50kg (approx) steel bars on to simulate the original 40kg fixture, the existing motor seemed to lift it. It wasn't exactly eating it for breakfast, nor was the centre of gravity quite correct, but it did lift.

This gave me some hope that by doubling or tripling the torque of the motor, it would solve the problem. But maybe lifting it once or twice is different to a full shifts worth of usage, and I had no idea about the amp issues.

So....

12 Volts does not seem able to give us more than 40 Nm Stall Torque, or 4 Nm real torque, when we need at least 12 Nm.

24 Volts does not seem to give us much better, looking around, and would require a whole new electrical control box anyway.

230 Volts is too high, apparently, because the base of the fixture rotates 180 degrees and the wire to the motor comes up the middle of the stand.

I think they are (rightly) worried that eventually the cable could break and be a hazard, even though it is rarely turned and the cable is quite slack. Then there is the issue of the different voltage for the actuator, which seems to come in either 12 or 24 volts only, not 230, which maybe equally dangerous even if there were any available anyway.

I had hoped that the much higher torque motor, either ""25Nm"" or ""40 Nm"", would salvage all of the work done so far and allow us to use the 12 volt actuator too.

It could have been worse, we could have ordered the ""25Nm"" motor and the 12 Volt actuator and wasted another £300 or so.

Well, I guess it is time to go cap in hand to the bosses tomorrow, to tell them the news.

Maybe the only way out is to attach some great big counter weights on it, but I don't think the customer will be happy with that, they are expecting it to be sufficiently powered on its own - and with there being different product weights, this may get tricky to find a good balance.

Thanks again for your advice, it is sad to know things are so wrong with everything, but at least I have not thrown more good money after bad. Best cutting our losses now rather than make it worse. This job is just snowballing.

Cheers.

 

[waross]

For intermittent use. You can probably stand some overload. Look for a motor of at least half of the needed 11 Nm.
7 Nm or 8 Nm would be better.
BUT, make sure you are looking at the rated torque, not the stall torque.

If you can double the gear ratio that will be the equivalent of boosting the 4 Nm to 8 Nm.
If you can live with the reduced speed.
Here is a possible candidate:
Mode #319.4862.20.00, 12 Volts, 8 Nm, 45 RPM, 6 Amps.

http://docs-europe.electrocomponents.com/webdocs/12f1/0900766b812f1f8e.pdf

I hope I get this right:
30:1 and 45 RPM = 1.5 RPM
1.5 RPM = 40 seconds per revolution.
40 seconds per revolution = 10seconds per 90 degrees.
The implications on the motor:
(11/8)[sup]2[/sup] = about 190% loading.
Well, for 10 seconds you can probably get by with that short term overload.
Fell free to point out if I have missed something.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

Thanks once again for your help and advise on this matter, it is greatly appreciated.

I have tried to explain the situation to those involved in this job here, and the consensus (at the moment) seems to be that we'd better not risk buying another motor that may not be up to scratch for production use.

There are apparently another two of these to make in the future, so it may be best to just swallow the losses on this one and get it right, then balance the cost over the next two.

We have just done a test lift on the original motor.

The "maximum efficiency torque" for that motor is 2.1 Nm. It has lifted the 42 Kg fixture weight no trouble, and quite speedily. However, we added a weight of 20kg to the fixture and it would not even entertain moving. It overloaded the control box and we had to use the reset button. My guess is that the original motor would lift 50 kilograms.

However, I think you're trying to tell me that even though it can lift this 50kg, it would not be suitable for 8 hours of use in a production environment.

The 8Nm motor may therefore do it after all......but I am still a bit uncertain about it.

My simple brain is a bit too fried to check the rotation time, but it sounds about right to me. Anywhere between 6 and 20 seconds is feasible.

I have found a motor spec on the DOGA website, which is much better and runs at 12 volts.

It is the 258.3712.20.00 model.

http://www.oem.co.uk/products/Motor...ors/DC_gearmotor_type_258/1159529-500261.html

The spec looks interesting, and I think it gives us 12 NM "normal" running.

It is 40 rpm, which is a bit slow, but as long as it is under 20 seconds or so, it may be okay.

I don't know how much they are yet. I am guessing at around £420.

I will need to discuss this further, to see whether it is worth the extra money, or to try the 8 Nm one for about £220.

What are your thoughts on the 258.3172.20.00 version?

Regards,

 

[waross]

The 15 Nm motor looks like a safe choice.
Be aware that your control device must be capable of handling the 55 Amp starting surge.
As a final check, check the current that the motor draws during operation.
The running current should be equal to or less than the rated 12 Amps.

Here are some comments on motor overloading and motor burn-out.
I hope that this is helpful to you when the question is asked:
"Well if the smaller motor worked once, why do we have to spend more money for a larger motor?"

Heat is the enemy of motors.
If the motor gets too hot, the insulation fails and the windings short out.
The motor has resistance. The heat developed in a motor may be expressed by the current in Amps squared times the resistance in Ohms.
The motor will radiate heat and lose heat by conduction and convection.
A balance is reached at a temperature where the heat lost equals the heat developed.
If this temperature is too high, the insulation fails and the windings short out.
A cold motor may be safely overloaded for a short time. But the time that the motor is overloaded must be shorter than the time it takes for the motor to reach a dangerous temperature. This must be followed by sufficient time at reduced load for the motor too cool down.
While it is possible to intentionally overload a motor for short periods of time, factors may change:
A motor may work well being overloaded for part of the operating cycle during the autumn, winter and spring but fail on a hot summers day.
The operating cycle may be changed by the operator to an operating cycle that no longer allows sufficient time for the motor to cool down between overloads.
You are considering a cycle of 10 to 20 seconds every 10 minutes. This is a good candidate for an intentional short term overload, or is it. An undersized motor may work well for years until the day that the operator, for whatever reason, runs 5 consecutive cycles with no cool down time and the motor fails.
Will that ever happen? Let's ask Mr. Murphy what he thinks.
I just got the memo back from Mr. Murphy.
He refers me to "Murphy's law" which states that this will happen and the motor will fail.
By the way, there are calculations for safe cyclic loading of a motor. In the Cowern Papers this is called "RMS Loading". These calculations may be applied to a machine where a motor is momentarily overloaded during part of a cycle. Some applications are punch presses and metal shears. These machines experience a very short momentary overload during the actual cutting or punching of the metal.
The cycle is known and is not subject to operator changes.
RMS loading will not be safe to use in your application. The operator may easily shorten the 10 minute cool down time to an unsafe interval.
Disclaimer: I am a Ft. Lb. type of guy. I don't generally work in Nm. I have accepted your Nm values without verifying them.
However, I accept BrianPetersen's values and your experience with the 2.1 Nm rated motor is further confirmation that we are in the correct ball park.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sirius2]

Once again, thanks for your insight and expertise on these matters.

The factory it is being installed in is very spacious and well ventilated. The English town it is in hardly ever sees any warmth or sunshine....(sigh!)...so environment shouldn't be a major problem. However, if they are exporting the two others to a foreign factory, it could be.

I am hoping that the intermittent use will be our saviour on this. From our rather selfish perspective, if it burns out in 4 or 5 years time, then the motor will have more than paid for itself and a new one could be installed. It is not a great attitude to take, I know, but we are up against it at the minute.

The operation is it doing could not be done any faster - it is somebody manually bolting steel brackets and things to an object. So it kind of takes 'as long as it takes', if you know what I mean. They would not be able to rive the hell out of it without being deliberately destructive.

We have contacted the motor suppliers for the 12Nm motor I listed last, and they are approximately £330, which is not so bad if it got us out of the fix......but the problem is, they are a whopping 10 weeks delivery. This has in effect ruled them out of the equation. The first fixture is due for delivery in about two to three weeks!

I have just been asking about the amps and the present control box. I am told that the present equipment cannot cope with the 55amps to run that motor anyway - and would apparently need significant re-working to make it do so. So again, that kind of goes against it, in sheer practicality terms.

In light of this, I have been told to order the 8 Nm version we discussed earlier. The thinking is that the 2.1 Nm worked fine on 50Kg. If we are at 8Nm, that is four times the torque that lifts 42kg (say 160kg). It may not work out that way, but we'd be much closer to the 120kg maximum they'd ever put on it. I may be able to figure out some neater looking counterbalances that look like they are part of the stand, if needed, to help too.

I would have preferred to change the capacity for the amps and wait for the higher spec motor....but the decision has been out of my hands now.

regards,

 

[waross]

As for short cycling;
That would be when the operator for some reason lowers and raises the load several times without actually doing any work on the workpiece.
It happens. Just ask Mr. Murphy. grin.
In your position I would go with the 8 Nm motor. It sounds as if your supervisor understands and is willing to take the risk.
You may consider the following, if time permits;
We know the net torque required but we don't know the torque required including friction.
Consider ordering one motor and measuring the current under actual operating conditions. The current profile during a complete raise-lower cycle may be important.
I expect that the current will start low and increase as the fixture rotates towards the horizontal position.
Once we see the actual current profile required, we may be confident with the 8 Nm motor, or it may become apparent that a larger motor should be used.
The current as a percentage of rated current should be a good indication of the torque as a percentage of the rated torque.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lukin1977]

I would not select a motor rating undersized on the account that it wont be damaged because it will only work 1 o 2 times per day, because things can change. For example, the frequencies of use may change or new people will arrive and do not know about this limits of usage