Axially Flexible Shaft Coupler

[mrpi]

I'm having a problem coupling a ball-screw actuator to a gearbox.

Attached is an exploded view of the assembly.
Note that the Worm Drive Gearbox is provided with a hollow output bore, the Custom Machined Shaft has a keyway and key on it to engage the hollow output bore.

The key feature here is that the Custom Machined Shaft is allowed to float in the hollow output bore. This is to allow setting of the air-gap on the Shaft Brake Assembly.

The air gap is set using the Clamp Nut and Spacer Ring. Once the air-gap is set to the specific brake and the Custom Machined Shaft is removed and Loctite-ed into the hollow output bore to fix it axially (permanently). Loctite 638 is used after cleaning and priming the surfaces. In this configuration, the Clamp Nut and Spacer Ring prevent the Custom Machined Shaft from moving to the left (in the image) and the Loctite prevents it from moving right.

NOT SHOWN is a machined block that connects the Worm Drive Gearbox to the Ball-Screw Actuator. Its just a simple block with a hole in it.

The Custom Machined Shaft turns the input shaft of the Ball-Screw Actuator by means of the Rigid Clamp Shaft Coupler.

The Rigid Clamp Shaft Coupler is tightened after the actuator and gearbox are bolted to the not-shown-block.

HERE IS THE PROBLEM I'M HAVING:
We load test the assembly by pushing on the end of the ball-screw actuator and supporting the gearbox end. We push to 20k lbf, which the actuator is rated to.

It appears that during loading, the Ball-Screw Actuator input shaft moves relative to the Custom Machined Shaft and thus transmits an axial load to it. Since the Clamp Nut and Spacer Ring are quite rigid against the bearings in the Worm Drive Gearbox, what slips is the Rigid Clamp Shaft Coupler (slides down either the input shaft or the Custom Machined Shaft). THEN, when the load is removed, the returning force of whatever deflected pulls on the Custom Machined Shaft. This breaks the Loctite bond and closes up the air gap on the Shaft Brake Assembly. This causes problems with the brake being always-slightly-engaged.

SO, obviously there is something deflecting when the load is applied. It could be parts in the Ball-Screw Actuator or it could be the Not-Shown-Block that smashes down. I am very reluctant to change either of these parts.

Therefore, what I am looking for is a torsionally rigid shaft coupler to couple the two 3/4" shafts BUT allow for some axial float. The axial float can occur on only one end, both ends, whatever. I'm having trouble finding something off-the-shelf and I'm open to suggestions or ideas. Torque transmitted by the coupler is around 1200 in-lbf. Speeds are very low, several hundred RPM. Ideally it would be corrosion resistant, plated or made from stainless/aluminum etc.

Sorry for the long post, but I just wanted to be clear on the mechanical setup.

I was thinking of maybe some kind of spine-type coupler?

Thanks!

Beat to fit, paint to match.

 

[MikeHalloran]

http://www.heli-cal.com/cm/Home.html

Mike Halloran
Pembroke Pines, FL, USA

 

[tygerdawg]

You say 3/4" shaft, so that gives a hint to the size of your rig. Heli-Cal would be my first thought also (kudos to Mike). But if you looked at several of the power transmission components companies (Nord, Falk, Eaton, Rexnord, et al) then you will see many different types of couplers that will do what you want. The challenge would be getting one of the proper size since most of those couplers are for heavier equipment.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[mrpi]

Hi Mike,

I browsed through the Heli-cal catalog, and the only thing I saw that looked like it would handle the torque was in their "power transmission" page and there were no details other than the one photograph.

Also, the angular position of the actuator (read: length) is critical in my application, so wind-up is a big concern between the shaft brake and the actuator input shaft. Although the heavy-duty helical couplers appear to have very high torsional stiffness...

I'm checking out the industrial power-transmission companies that tygerdawg mentioned.

Thanks for the replies!



Beat to fit, paint to match.

 

[diskullman]

A mini gear coupling will allow for some axial float

http://www.ktr.com/en/products/couplings/bowex/910_junior.htm?frame=us

Russell Giuliano

 

[chief]

Sounds like what you need is a quill shaft drive,commonly used in steam turbine transmissions, but I am sure there may be a smaller version available for your rig.You will find more info available on this forum with a search.

Offshore Engineering&Design

 

[Ron]

Are you considering a spline connection similar to a transmission-to-drive shaft connection in a rear drive vehicle? That connection floats axially.

 

[unclesyd]

I would give R-W Couplings a call.
We used a lot of this type couplings in our machines due to misalignment and frame stiffness problems.

http://www.rw-america.com/miniature_couplings/index.htm

 

[mrpi]

Hi Russel, I looked at those mini-gear couplings and they look nicely made. However, they're also quite expensive. I might have to fall back on something like that though.

Chief, I'll have to look into quill shaft drive. I'm not familiar with it.

Ron, I think something like a spline connection would be ideal. I have used them in other applications and they're worked nicely. I was unable to find an off-the-shelf retailer of spline couplings. I may look at having parts machined for my application.

Unclesyd, I actually called up R+W and spoke with an application engineer and he said what I'm looking for is right at the ragged edge of their couplers can provide.

I'm looking at having ZeroMax make me a high-torsional-stiffness composite-disk coupling. In another project we use a custom-made floating drive shaft that they make for us and it works very well.

Beat to fit, paint to match.

 

[diskullman]

Perhaps this brand is cheaper

http://www.huco.com/products.asp?cat=69&p=true

Russell Giuliano

 

[BrianE22]

"Double Disk Couplings" are soft axially and stiff torsionally.

 

[unclesyd]

We used metal version of this type coupling under some very demanding conditions. In our coupling the shaft came through the barrel so we could put shim to set the axial travel.

http://www.techdrives.co.uk/html/barrel_gear_shaft_couplings.html

 

[unclesyd]

Off the wall information. Could you simplify your drive to one like shown in the Candy link. The next two links are for splined shafts and hubs for possible incorporation in your drive. You may want to look a Hayes Jaw Coupling.
Last but not least is my favorite shopping page. They make the jaw couplings with a splined hub that isn't a split locking type. As they are half couplings you can have different hubs with nothing special.

http://www.candycontrols.com/coupling/Candycoupling.htm

http://www.grobinc.com/coldrolled/

http://www.huco.com/index.asp

http://www.hayescouplings.com/index.php/products/original_jaw_couplings#catalog

http://www.surpluscenter.com/item.asp?catname=powerTrans&item=1-2008

 

[diskullman]

These spline bored couplings are not intended for a slip fit on the plined shaft, as found in many automotive and agricultural application. They are mostly intended to clamped tightly to a splined shaft on a hydraulic pump applications. Magnalloy couplings are very popular with splined bores, and the cheapest splined bores are found in Lovejoy jaw couplings, but rarely stocked by distributors. They are usually factory stock.

Russell Giuliano