Mech. Engineers..please help.

**Darkman** · 20th April 2007, 23:17

I have a simple beam problem that i am having problems with. just a motor-gearbox assembly on a beam. i have to find the Z of the beam to choose a suitable size RHS. So i need to calculate the max moment on the beam, so need the reactances at X1 and X2. To do that I need to work out the forces on A,B,C and D. I have the weight force of the objects, and the centroids. My question: How do i calculate the amount of force of the shaft that acts on A,B ,c and D? I have gone and used the the following formula and assumed the all the forces on the shaft acts only on B and C ..F1xL1 = F2xL2.
What is the correct way to do this?

please excuse the good picture.

**Grub** · 20th April 2007, 23:27

Sorry to hijack your thread D-M but even though I have no idea what that is, I like the purdy picha

**merv** · 20th April 2007, 23:31

You've missed telling us the total length of the beam and/or the distances between X1 and A and D and X2.

**Darkman** · 20th April 2007, 23:48

Originally Posted by merv

You've missed telling us the total length of the beam and/or the distances between X1 and A and D and X2.

total lenth of a beam is 4meters...i did not give all the distances cause i only need to know how to go about solving the problem.Will try to add them in my pic

**cowpoos** · 21st April 2007, 00:53

Originally Posted by Darkman

I have a simple beam problem that i am having problems with. just a motor-gearbox assembly on a beam. i have to find the Z of the beam to choose a suitable size RHS. So i need to calculate the max moment on the beam, so need the reactances at X1 and X2. To do that I need to work out the forces on A,B,C and D. I have the weight force of the objects, and the centroids. My question: How do i calculate the amount of force of the shaft that acts on A,B ,c and D? I have gone and used the the following formula and assumed the all the forces on the shaft acts only on B and C ..F1xL1 = F2xL2.
What is the correct way to do this?

please excuse the good picture.

I fucking big thick beam!!!! or two

PM dover [he knows this sort of shit]

**Brian d marge** · 21st April 2007, 02:01

One assumes that the shaft need to be kept parallel to the motors , so there is going to be a deflection involved , just go back to basics, , freebody , slope dia , moment. shear etc sum of the forces is zero , sum of moments etc ( sorry just read the question again .. the weight of the shaft is split between both motors causes an up ward force and a down ward force on the motor mounts ,,a careful freebody will show the direction of the forces....)

My guess without knowing the question is that the shaft needs a max angle , this will give you the deflection and the Moment of Inertia

or they would have given you max stress , or factor of safety or something like that

Stephen

**Drum** · 21st April 2007, 09:07

Darkman, do you mean that you want to add the equal and opposite reaction to the shaft torque to the beam calculation?

As the shaft is rotating along the axis of the beam, it won't add any moment to the beam (apart from its dead weight), but you may need some diagonal bracing to the side of the beam to resist the rotation.

To determine the forces at A, B, C and D assume pin joints at these points and distribute the downwards force according to the distance of the centroid (motor, shaft and gearbox) from the support point. Then, assuming a pin joint at each end of the beam, sum the moments to zero to determine the reaction force at either X1 or X2.

Don't forget the dead weight of the beam itself, and add use a factor of safety of at least 1.5.

Are there actually two beams, running parallel to support the motor and gearbox? Lateral bracing between them will help resist the rotation force.

**merv** · 21st April 2007, 09:49

Hi just got back to this - Drum has kinda summed it up - my background is Civil Eng not mech and we always applied live loads at 1.7 load factor but I figure with your setup you probably should be even more conservative than that to keep deflections and vibrations to a minimum. Are there any universal joints on the shaft? If not you don't want the beam deflecting much at all.

Calcs based on pin joints like Drum says of course are conservative also for moments in the beam.

Are you OK with all this now?