Design of rotating shaft with axial load

[evilspeeder]

Hi all, I'm trying to design a rotating shaft for a large piece of machinery. It is supported by two bearings with a scoop mounted to the middle of the shaft between the bearings. The scoop is perpendicular to the shaft so that as the shaft rotates the scoop sweeps material away. It is driven off a gearmotor attached to the shaft at one end (outside the bearings). The shaft will obviously see torsion from acceleration and braking of the rotating mass, and it will see fully reversed bending from the weight of the scoop and it will also have an axial force and moment from material hitting the side of the scoop (parallel to the axis of the shaft).

This axial force is what has me stumped. I don't think I can use ASME B106.1. Should I use that to get it close and just check for buckling after? Is there a separate method for a shaft with complex loading?

Thanks,

John

 

[zekeman]

Put some numbers in so we have a clue as to the dimensions of the problem and don't worry about codes just yet.
What are the weights, forces, distance between bearings, rotational speeds, expected life of the system, etc?
Maybe that is why you haven't gotte any responses.

 

[evilspeeder]

Good idea.

From the motor end: 10 in of shaft, then a bearing, then 10in of shaft and the scoop, then another ten inches and the second bearing. I'm planning on attaching the shaft to the scoop using two keyless locking hubs. The motor/gearbox will be foot mounted and connected to the shaft with a flex coupling.

The shaft will have to transmit about 1600lb*ft of torque to the scoop to accelerate and decelerate it. The shaft and scoop assembly weighs about 1400lb with the majority of that taken up by the scoop itself. There will be a force on the scoop of about 1000lb creating both a bending moment in the shaft and an axial force. This moment is about 2000lb*ft.

The assembly will stop and start every revolution and must be highly reliable. The acceleration and deceleration require the most torque, the actual scooping operation is negligible.

For now assume no unusual stress concentrations, material is steel (haven't selected an alloy yet).

In past designs these shafts have been about 4" in diameter.

This is something I'll have to do a lot, and some of the shafts can get somewhat complex with a number of steps and stress concentrations, is there a good piece of software for this short of full blown FEA?

Thanks,

John

 

[evilspeeder]

I forgot to add, max speed is 45rpm and est hp is about 14hp.

 

[zekeman]

First off, you don't have a buckling problem with a 4" shaft supported on 2 bearings 20 " apart.
The bearing(s) have to be designed to take the axial load and by superposition you solve for the stresses due to the moment , the compressive force due to the off-center axial load and the torsional stress due to the acc/dec of the inertial load. Keep in mind that the off-center load manifests itself into the axial force and a couple.