Loads on a system 2

[joesm519]

There is a reducer box whose output shaft drives a conveyor belt. The outrput shaft is off-center (closer to one of the short sides of the rectanglular face of the reducer box). The other end of the reducer box is supported by a vertical rod that is bolted into the ground. I am doing stress analysis of the reducer housing. The weight of the reducer box is the only load I have in my linear static FEA. I did not include the output shaft torque in my FEA since the shaft is free to rotate. My dilemma is that during startup or when the conveyor belt load varies, the reducer box would jerk a little. I am thinking of accounting for these loads (due to the sudden jerk) in dynamic analysis. When the system jerks does any of the torque come into picture in the static stress analysis? I am thinking that the only way the torque will have an effect on the linear static stress analysis is due to friction in the bearing whereby the torque is converted into a load tangent to the output shaft. please let me know if I am on thr right track or missing something.
Thanks

 

[FeX32]

I believe you are on the right track. From your description this sudden jerk is a result of the inertial load of whatever you are driving. This would not effect the static analysis you stated. However, it does create an extra load on the system during the initial transient.
In your picture,it seems that the inertial load acts as a moment with center at the center of the shaft. Your support is at the other end of the box and thus this is probably why the box shakes. Also, you said the load varies, so I am thinking that maybe you can do modal analysis on the box to find our what your mode shapes and nat. freq's are. This can tell you if you need to increase the stiffness of the support possibly to reduce vibration. Or maybe you can increase the compliance to increase the damping of the support.
Just some thoughts,



Fe

 

[joesm519]

Thanks FeX32,
In the static analysis I went back and included 5% of the torque to account for the friction during startup. The reason I did this is, suppose there were no support at the other end and the entire reducer box is free to rotate about the output shaft. During startup when the friction is max then the shaft would act as if it were 'gripping' the box and try to rotate it in the same direction as the shaft. So I applied 5% of the output torque to the box as one of the loads. Now I dont have any basis for the 5%. I found online some friction coefficient values for a roller bearing as .005. Do you see anything wrong with the above analysis.

I did a modal analysis on the housing without the internal gears and shaft. In cases like these does one have to model all the 'guts' of the system. If so then is the modal analysis of the housing (without the gears and shaft) a total waste?

Thanks again.

 

[FeX32]

Glad I could help.
I don't see anything wrong with your reasoning. For your static analysis 5% is a reasonable number to assume for the friction.
However, if you are going down this road are you including the inertial torque as well as the frictional? I am not sure if what you are driving has large inertia or not. If during your initial start up you have a lot of acceleration then you may want to consider this because this torque component can be larger then the frictional torque.
In regards to your modal analysis it is not customary to include the 'guts' as you say.
ttyl,


Fe

 

[EdDanzer]

You have what is called a torque are reducer and all the rotational torque will transmit to the anchor through the box. This includes starting and stopping torque spikes which can be much higher than running torque.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[EdDanzer]

You have what is called a torque arm reducer. Sorry for the typo.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[FeX32]

Nice, I never knew there was a name for it.


Fe

 

[joesm519]

EdDanzer and FeX32,
Thanks for the reply. Does that mean I need to apply not the frictional torque of 5% but the total torque(or more)? Initially I had applied the total torque on the inner surface of the reducer box through which the output shaft goes. The stresses went sky-high but there is a unit that is functioning well. So I made the assumption that may be only the frinctional torque is contributing to the stresses on the reducer housing. The one difference between the actual system and my modelo is that I am analyzing only the housing and that is all my FEA model has. I dont know how much stiffness the gears and the motor-shaft are contributing in the actual system. Any thoughts?

Thanks

 

[FeX32]

I think it would be overkill to design the box assuming the maximum static torque applied to the box is the maximum torque of the motor, as this is not how it should function. The inertial torque I mention is the torque that contributes to the angular acceleration of the components and driven parts, better known as T=J*alpha in rigid body dynamics.
T=inertial torque
J=polar mass moment of inertia
alpha=angular acceleration

This is the main torque that is applied to the box (with center at the center of rotation) during the initial transient stage (ie. the initial start-up)

To start you off.
I would first figure out what your average angular acceleration during start-up is. You can do this even crudely with a stop-watch if it is not known.
Then, you can approximate the polar mass moment of inertia of the rotating parts.
With this information you should be able to get an idea of the inertial torque that is applied to the box during this initial transient.
It is likely to be higher then that of the static frictional torque that you mention.
This is the real torque that is applied to the box when you experience the 'jerkey' start-up.
Hope this helps,



Fe

 

[EdDanzer]

The best way to model this may be to fix the bore and apply the load like a torque wrench because that how it functions. If you have electric motor power, or a brake the starting and stopping torque should include the stopping inertia of these items.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[joesm519]

FeX32,
"...Then, you can approximate the polar mass moment of inertia of the rotating parts".
I think you mean polar MI of the ALL rotating parts gears, shaft + the box and the over hanging motor which I did not show in the sketch. The motor is over-hanging to the right of the vertical support i.e. it is to the right side of the vertical support in the image posted before.
I dont have access to the system so getting the angular accel will be difficult. I do understand that I have two variables now T and alpha. Unless there is an alternate way to calculate alpha I might have to make assumptons.

Thanks for your help

 

[BobM3]

Is the motor body attached to the gear box or is the gear box connected to a motor with a coupling? If connected with a coupling you also have to include the motor's torque as an input to the gearbox.

 

[FeX32]

If the motor is hanging to the right of the box the inertial torque has its center at the center of rotation of the motor.
Eulers law is for rigid body dynamics only. So, the box should not be included in the polar mass moment of inertia calculation.
And, yes I do think that if you don't have access to the setup then you do have to make some approximations. Maybe you have a spec sheet for the motor and/or components?
all the best,

Fe

 

[joesm519]

I have attached a more detailed sketch which shows how the reducer box, coupling box and the motor are configured. The output shaft of the reducer box drives a conveyor belt. My primary concern is the stresses in the reducer box housing. My loads in the static analysis include the weight of the system and 5% of the output shaft torque.

Thanks

 

[BobM3]

Sorry, I'm a bit confused. You're looking at the stresses in the gearbox housing when the motor is driving the conveyor belt, correct? If so, then you have torque inputs from the motor and the output shaft plus possibly radial and axial loads from those two shafts plus the reactions at the support. Why are you only looking at inertial loads and gearbox weight?

 

[joesm519]

BobM3,
The output shaft of the reducer box is driving the conveyor belt shown in the sketch as a big red circle. The motor shaft supplies power to the reducer. Thanks

 

[BobM3]

That's what I assumed you had. Are you looking for static or dynamic loads? You do know that unless the input and output torques on the gearbox are equal (1:1 gear ratio), the gearbox will have to react the torque difference? i.e, if the input torque is 100 in-lbs and the output torque is 1000 in-lbs then the gear box housing reacts 900 in-lbs of torque. That reaction torque is probably what produces most of the stress in the gear box housing.

 

[joesm519]

BobM3,
I am doing a static analysis. My dilemma is that during the start up the torque loads are due to
1. The friction in the bearing
2. The inertia
3. Reaction torque
or all of these combined?
The output torque on the reducer is 18 times higher than the motor torque. When I apply this torque the stresses are sky high. Even if I apply 17 (18-1) times the motor torque the stresses are tremendously high. These are not stresses at singular regions. That is why I was thinking that may be during running conditions the torque may not have any affect on the housing and during the startup the torque might only be because of friction and inertia. The reaction you are talking about will be in the shaft. The shaft (and other rotating parts) is not rigidly connected to the housing but riding on the bearings so the housing itself may not see the entire reaction. Right? If so then how much?

Thanks

 

[BobM3]

It's a right angle (not inline) gearbox so I'm wrong in saying the gearbox housing reacts the difference between the input and output shafts. It fully reacts the ouput torque about one axis and fully reacts the input torque about an axis 90 degrees from the other.

 

[EdDanzer]

Your motor will generate the starting torque to overcome friction and inertia. The anchor of the gear box is what sees the reaction torque.
Accurately modeling the stress is the actual housing could be difficult as it will be a non linear assembly. A simple solution is to fix the anchor point and apply the torque to the bearing bores of the output shaft. The results will be higher than a complete non linear where all the gears and bearings are included but much faster and simpler to setup and run.
The output torque will be the gear ratio time the motor torque. Depending on the motor this can be very high. Many time the gear box will come loose or break from starting and stopping loads.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com