Torque to load in conveyor application

[CavemanJones]

Hello guys, i have a question reguarding my resulting force on a shaft that a motor is driving.

See attachment for clarification.

My issue is calculating the total load. I would then use that determined load and use it in a beam calc between the bearings. Static - Lets say my arm for my motor is 4" and my distance from the arm to the center of gravity to my motor is 8" and my motor weighs 30 lbs. I determined static load by 30lb * 8in = 240in/lb divided by 4in to get 60lbs. Dynamic lets say my motor outputs 800 in/lb of torque. I take 800in/lb divided by 4in to get 200lb. This would be the load at the end of the torque arm. What would be the load on the shaft itself linearly? When I calculated this before i just took my max 200lb (dynamic) + 60lb (static) = 260 lb max load. This cant be 100% acurate but if i were to figure worse case this would be it right?

Thanks for your guys help

 

[zekeman]

Would you please reconfirm the distance from the bolt to the shaft, L,.
From your 1st drawing it does not look like 4 inches-- looks like motor length plus 1/2 the reducer + a link to the bolt-- to me more than 4 inches.

As I have told you, the dynamic force is equal to the torque , 800 in lb divided by that distance, L, I'm asking you to get. It's simple physics.

You might also revisit your static result, since your explanation of having the motor CM on the other side of the shaft doesn't make sense to me. From your drawing, it is clear to me that the static force must be less than the weight of the motor/gearbox.
As a final note, you should add a dynamic safety factor of at least 2 for the dynamic force. Also,another reason that supporting the motor with the driven shaft is not a good idea is that,vibrations coming from the motor dive could resonate through the shaft.

 

[CavemanJones]

I didnt get to talk to my SEW rep however he sent me a PDF of mounting material.. i didnt get to look at it but he said they dont recommend driving a shaft between bearings.. i will find out exactly why tomorrow.

Yes its a very small box.. its only about 5 inches in height and the arm is 4.33

the true center of gravity isnt 8" out. but i know in this case if it was it would be less thus my static load would be less.. im taking this into worse case senario's.. reason center of gravity is not acurate is because the motor is a solid mass.. so thats how solidworks generates it.. im sure its more like 5" or 6" in realtion to the end fo torque arm but again, if i consider 8" then it results in a higher load when calculated.

Yes i agree with you dynamic load would be 800/4.33 = 185lb. this is on the reducer torque mount. right?

I was trying to determine a reaction fore of that load if any onto the shaft in a linear motion.

 

[desertfox]

hi CavemanJones

I'd listen to the SEW rep like your alluding to and I agree with him, if you re-read my first post its not a good idea to support the motor on the shaft its supposed to be driving.
Once you have established your mounting position for the motor and you need further help feel free to come back.
PS let us know how you get on.

regards

desertfox

 

[Ross0684]

Hi Caveman
I have come into this discussion a bit late.
However, we build many conveyors in a given year using SEW and other brand hollow shaft gearmotors with torque arm mounting.
I surmise that this application is for a lift up transfer in a pallet handling system and would guess that the conveyor only runs in the raised position??? Service factor would be well above minimum recommendations.
I have never had any ongoing issues with mounting a drive in the manner you have shown-perhaps you could move the torque pick up point to the underside which will take some of the static weight as well as place the gearmotor as close to the bearing as possible.
Isn't any torque generated by the motor reacted directly by the torque on the sprocket driving the chain(which in your case is very small)
I have had issues with gearmotor suppliers trying to prove to me that the conveyor design is too light to handle the capability of a selected gearmotor but they lose sight of the fact that the gearmotor will only generate enough torque to drive the load ( unless of course the conveyor can drive the load into a solid structure and this is easily controlled by the use of torque limiters or thermal overload protection).

A good test to satisfy your mind is to briefly start the conveyor with a spring balance in place of the torque arm mount and you will be surprised to see how little reaction there is being generated.

With regard to the SEW SA 37 range, one of the downfalls of this box is the small shaft size available.
If they are available have a look at the WA series if you can get the necessary ratio as they are available with a 25 mm (or 1" I guess in the US) bore.
Regards
Ross

 

[CavemanJones]

Thanks for your post Ross, Yes its a 3/4" shaft which is small.. Right now my rep says the W box's are having longer lead times (here in america maybe? or just here in missouri) of like 12 weeks.. So i had to design in a S box for that reason. The W box's are pretty nice for the cost.

Well, the reason i have started this thread is because i was questioned on the design as it was built. I did the calcuations above and found it to be okay. It is built and the motor is mounted as we speak. However it has yet to be run. I will be sure to let all of you know how it turns out and what i need to do. Meanwhile i will talk with my SEW rep because i have mounted reducers between bearings like that ALL the time. Im not sure what affect that would cause which is why i will contact my rep and post some info that we discuss as well.

As a side note, yes the motor is over rated. When we switched to SEW my boss wanted us to use them for their engineering and have them spec our motors. I have come back with possibly using a 1/4 hp and it would be just fine. 36 * 1/4 = 9 lbs out * 51 reductions = 459 * .9 (eff) = 413 in/lb. I am using a 17 T sprocket = 4.082 PD 2.041" radius. 413/2.041 = 202 lb out. Not to mention my load is running on roller chain. Well above even with a 1/4 HP

I will keep you guys posted.

Thanks for the replies!

 

[zekeman]

Ross,

Maybe you can tell me from what you know, what this so-called "torque arm" is.

Is it the connection from the bolt pivot to the centerline of the hollow shaft ( i.e. the output drive shaft), or what? Or better still, tell me what you think is the distance between the bolt and the hollow shaft.

I suspect that I'm missing something here, since my take on this is that independent of what you call a torque arm, the only thing of interest here is the distance between the bolt pivot and the hollow shaft.Are they the same?

And where is this so-called "torque pickup point" located?
Isn't it at the hollow shaft?

 

[CavemanJones]

Zeke, i will post the information SEW sent me on their mounting tomorrow.. it might give you a better idea of what it is.

Its bolted too the side of the reducer with 4 bolts. Its hard mounted to the gearbox and the arm extends out from center to center 4.33". It has rubber inside the end of it so it has some flex at the end of the arm where it is hard mounted with a bolt as shown.

Talked to SEW rep, actually had him in my manufacturing plant to examine the part. He said they dont recommend putting reducers between bearings because of the shafts not being perfectly straight. It doesnt hurt but it can put wear on the shaft. As long as the motor is allowed to float (as it does in this design) its okay. He looked at the application and claimed it would be just fine but its not typically recommended. He suggested using a flange mount motor on that side as a substitude for the bearing if possible.

All fine and dandy but still doesnt solve my original thought of how much load gets produced by driving the motor. I wish i was able to do some testing like ross recommended but i dont have the time at work to set that up (or the tools.. lol)

Thanks for posts guys!

 

[desertfox]

Hi CavemanJones
Well I was waiting for the mounting details before posting how it loaded the shaft or otherwise, however going back to your original post I imagined from the information you gave that the shaft would see bending and torsion, bending from supporting the mass of the motor and torsion from the operational load.
So you could calculate the shaft stresses using a Mohr circle for example.
A starting point might be this site:-

http://www.roymech.co.uk/Useful_Tables/Drive/Shaft_design.html

I also loaded a file for your reference and I think you got the 60lb static load on the motor by envisaging it the way I have in my sketch, anyway its just for reference.
Perhaps when we get the true mounting details we can give a better picture.

desertfox

 

[CavemanJones]

Yes fox.. However i think by the driving of the shaft there is some additional force being applied when its driven along with the 60lb load you pulled up.. would there be a reaction on the shaft whent he motor drives its 800 in/lbs of torque into the shaft?

I will spend sometime examining the site you posted.

See my attachment on mounting.

 

[desertfox]

Hi CavemanJones

I can only see a bending load from motor mass, the torque will be governed by whatever your driving, so other than that I can only see the loads as per my sketch.
Have a look at this link:-

http://www.freestudy.co.uk/d209/t9.pdf

go to page 19

desertfox

 

[Ross0684]

Hi Caveman
I think your SEW rep should have another think about what he said:

""dont recommend putting reducers between bearings because of the shafts not being perfectly straight.""

If it sitting on the shaft it must be straight-the flexible mount within the torque arm will take up any discrepancies between the mounting shaft and the torque arm pick up point.

Also:

""He suggested using a flange mount motor on that side as a substitude for the bearing if possible.""

This then requires the plate to which the gearbox flange is attached to be perfectly square to the centreline of the monting shaft-not easily achieved without introducing machining of the conveyor frame into the equation(with substantial costs).
This is the very reason torque arm mounts are utilised.

After over 45 years of being in conveyor manufacture I become very cynical about some of the advice equipment reps hand out. Granted SEW give their people good training but reps should also listen to their customers and the practicality of actually manufacturing something.

I will get down off my soap box now.
Regards
Ross

 

[CavemanJones]

Thanks ross.. I agree a flange mount would be a total pain in the ass.. lol

Like the document that fox posted says how if the bearings arnt aligned the shaft can flex. I suppose thats why they dont recommend using it between bearings because it can flex.. not sure why as you said the torque arm takes misalignment up.

Im going to have to review the documents you posted fox and get a better understanding of them.. I will still post the results of the design for everyone who might like to know how it turns out.

thanks guys!

 

[Ross0684]

Hi Caveman Jones
Below is the details of what I was trying to say

"The torque arm concept is the most preferred, since it enables a hollow shaft reducer to hang from
a solid shaft and to be totally supported by that shaft. A torque arm is simple to install and guards
against potential misalignment caused by foot mounting or flange mounting. When correctly
installed, the reducer experiences zero binding and zero overhung load, even with a load shaft that
is out-of-round. In fact, it is quite common for the reducer to slightly wobble during operation."

ie if there is any flexure in the shaft the flexible mount within the torque arm absorbs it.

Also if you are using self aligning bearings( which I would be very surprised if you wern't), the bearings automatically become aligned.

Ross

 

[zekeman]

OK. Caveman, got the picture finally and I concur with the 60 lb static you and Fox got.

Now, as for the dynamic force load on the shaft, it is precisely what you got in your first post, since the torque delivered by the hollow shaft must be taken by a couple whose
"arm" (distance from bolt pivot to hollow shaft)is 4.33 inches and so
F=800/4.33=185 lb
In addition,there is a small ignorable twisting torque on the main shaft due to the angularity of the line from the bolt centerline to the gripping point of the hollow shaft.

Both ends of the torque arm also have 185 lb of force but the end connected to the reducer carries the 800 in-lb of torque to be delivered to the reducer housing. It is not a requirement that the torque arm be located as shown; it could be anywhere on the reducer housing and will produce the same dynamic 800in-lb of torque and 185 of force on the main shaft.


Another way doing this to consider the whole assembly rigid with equal external forces on the "link" at the bolt and the hollow shaft and the 800in-lb of torque delivered by this "link" which is a solid connection starting at the bolt and ending at the hollow shaft.

The total load is the static force plus the dynamic torque and force on the main shaft.

 

[CavemanJones]

Very good, im glad you agree zekeman. I thought this would be correct but i wasnt sure. Does anyone else understand this concept that we have agreed too?

Fox, havnt reviewed those documents yet but will at some point. I appreciate you posting them for me.

Ross, yes your assumption would be correct, i would be nuts not to use self aligning bearings lol. Yea when i said misalignment thats what i was refering too. I suppose i should have put it if the shaft wobbles the torque arm allows the reducer to move and prevent it from taking that load.

Thanks for the posts. Been helpful!