Hub re-balancing after keyway modification...? 1

[sry110]

I have a situation where we manufactured and shipped out a steel rigid hub for a high-speed shaft (6000 RPM) with a 3/8" wide x 3/16" deep keyway, but it should have been a 5/8" wide x 5/16" deep keyway. Now the hub is in the Customer's hands and we are trying to find the best way to proceed. When we manufactured the hub it was fully machined and had the keyway cut, then it was sent out to be dynamically balanced.

So my question is: will it be sufficient to allow the customer to enlarge the keyway to the required 5/8" w x 5/16" dp. dimensions without rebalancing it? Or is it always necessary to check the balance after this type of mod?

My concern is the "air gap" between the top of the key and the bottom of the keyway, which is a thin strip of "missing" material. By making the keyway wider and deeper, the missing material is farther away from the axis of rotation, so it seems that any eccentricity would be reduced, and therefore increased imbalance and resulting vibration would not be an issue. But if the small bit of metal that was removed during the original balancing process was done to conteract the 'missing' material between the key and the keyway, then I could be upsetting the overall balance by modifying this keyway right? Or is the difference so insignificant that it would never manifest itself as a nasty vibration?

Am I thinking on the right track here, or am I just grasping at straws to avoid re-balancing and therefore reducing the amount of backcharges that I will receive from the Customer

EDIT: To put this in perspective, it is an 11 lb steel hub mounting to the end of a steam turbine rotor that weighs approx. 2000 lbs and rotates at about 6000 RPM at full speed.

 

[MikeHalloran]

At 1200 rpm, I wouldn't bother.

At 6000 rpm, I'd get it rebalanced.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

I agree.

 

[tbuelna]

sry110,

Was the hub balanced on its own, or as part of a rotating assembly?

As to whether the dynamic unbalance that may result from the keyway modification will have any adverse effects on the rotating assembly, that depends upon how well the supporting bearing system can tolerate the dynamic unbalance forces. The class of dynamic balance is usually selected to achieve a certain maximum unbalance force based on things like rotating mass and speed.

Whether your part needs dynamic re-balancing depends upon what class of balance the part is required to meet. Regardless, a re-balance shouldn't cost much or take long.

Good luck.
Terry

 

[diskullman]

Use a step-key check McMaster catalog, page 3332

Russell Giuliano

 

[sry110]

Thanks everyone for your input.

@ MikeHalloran: Typically I would follow the same rule of thumb for a new part...just wasn't totally convinced it was necessary when modifying the keyway only.

@ tbuelna: Yes, the hub wil be sweated onto the end of a steam turbine rotor assembly, and presumably the entire steam turbine rotor assembly will be balanced prior to operation (although that is in the Customer's scope of responsibility). Our balance critera is 4W/N, which in this case came out to 0.006 oz-in.

Also for whatever it's worth, my original post had bad information. The turbine shaft speed is just under 7300 RPM, not 6000 RPM as I wrote originally.

@ diskullman: Thank you for the suggestion. Unfortunately though, I need all the key strength I can get for the particular application. This is a retrofit job and I've been "designed into a box" from the get-go as far as the existing turbine shaft diameter and key size/length, how much torque they can carry versus how much torque I need to transmit through the system.