# Increasing torsional-stress resistance of an Auger at its motor-shaft joint

An auger is used to move plastic balls and is rotated by a stepper motor. Due to the balls getting jammed sometimes, this induces stress on the auger at the motor-shaft junction and breaks. Images below describe the issue better.

Specification:

1. Auger - 3D printed PLA
2. Motor - Nema 17 (4.2kgcm torque)
3. Plastic Balls Weight - 3 grams
4. Auger RPM - 180rpm

Possible solutions that I've thought:

1. Adding epoxy at the fracture section to strengthen it
2. Increasing the fillet diameter at the fracture section

Are there other options to solve this?

• Is it possible to add a photo showcasing more clearly the fracture type? May 13 at 9:27
• Does the motor have the necessary protection circuitry to sense that it's stuck and turn itself off? If not, you need the shaft to break to protect the (presumably more expensive) motor from burning out. May 14 at 14:42

As your construction is 3D printed, increasing the strength at the point of current breaking will more likely transfer the damage to a new location. The obvious and possibly impractical solution is to solve the jamming problem, rather than to try to power through it.

If it's not practical to prevent the jamming, consider to convert the jamming related breakage to a slipping solution. Engineer a coupling that depends on friction to perform the power transfer from the motor to the auger. Ideally, it would be adjustable, perhaps via compression screws or springs, allowing normal operation with no slipping until a jam is created.

Two disk shapes of metal secured to each shaft, in contact with each other provides the friction for power transfer. A clamping ring that only screws to itself keeps the disks together, but when the output shaft ceases to turn due to jamming, the ring allows the input shaft to continue to rotate.

If this is not monitored, friction will eventually deteriorate either the clamping ring or the metal disks, but it will prevent shaft destruction if properly constructed.

Build the auger around a stronger shaft which has a better connection to the motor shaft.

Perhaps a carbon fibre or even a steel centre.

But then consider what will be the next breaking point.

Or make a joint between the auger and the motor that fails if the mechanism gets jammed - just make it easy to replace.

You might consider dealing with it in software. Sense the stepper motor current, and when it exceeds a certain threshold that indicates jamming, perform a high speed reverse rotation for one or more revolutions. Then proceed in the forward direction.

For me a good (compromise of a) solution would be to redesign the shaft with an rectangular slot (although I am still unclear as to the fracture type, so I suspect is torsional). Like so

Then you can use a rectangularly shaped metallic shaft along the length, which will be able to transmit more gradually the torque.

*Figure 2: rectangular shaft image source: Engineering Library

Ideally it would be best if you could find a shaft with lateral "wings" like the following,

and the shape accordingly the shaft.

It would have a better rotational modulus and would not be subject to warping.

Additionally, since you are 3d Printing, you would benefit the most if you printed the shaft vertically (and not along its length). I.e. the axis of rotation in perpendicular to the printing plane.

In machine tools where an auger is used to remove scrap metal chips, the current in the auger motor is sensed. If it exceeds a certain level due to jamming, the electronics temporarily reverses the auger direction which can clear the jam. Lubrication of the material-auger interface is very important with augers, which have a huge swept surface area.

If you know the torque, or a close approximation of it, you can size the auger shaft more appropriately. From Machinery's Handbook the max torque T a shaft can handle is T= S_S* Z_P

S_S = allowable torsional shear stress of material
Z_P = polar section modulus (0.196 * diameter^3)


Make sure your units are consistent.

So you can either use a stronger material as Solar Mike pointed out, or increase your diameter in your proposed solution #2, but along the length of the shaft as needed.