Involute Spline Alignment

[ALREMAN]

I have a spline on a hydraulic motor that is a Flat-Root Side Fit per ANSI B92.1, 30 degree pressure angle. I am using the motor to drive a chain drive. The sprocket is a double sprocket that has two set screws, 90 degrees apart to secure it to the shaft. This is a low cycle, high torque, slow speed (<20 rpm) application. Since I am not using standard materials for the mating sprocket (SS 316), and I am exceeding the recommended max bore size for the sprocket, I am going through the stress calcs to make sure my drive will hold up. I am basing my calcs off of the Machinery's Handbook approach. I see that alignment is important and depending on what load distribution factor I choose (Km) from the table my results can change dramatically. I am looking for some guidance on how to choose the correct misalignment factor. If I take the diametrical clearance and divide that by my engagement length, I get .033"/1.60" = .021 in/in. Is this my max misalignment that I should base my load factor on? The table only goes as high as .008 in/in. Alternatively, should I select a misalignment factor for the design and use that as alignment criteria for installation? I think that a direct mounted sprocket with a side load (chain tension) will behave differently than if I had two shafts coupled together with some angular misalignment. My two set screws, when tightened down, will cause some eccentricity between the drive shaft and the sprocket center. Additionally, the sprocket will always ride on the high tension side of the shaft due to the chain. I picture this causing wobble at the end of the sprocket where the set screws are. Does anyone have some insight here?

 

[tbuelna]

ALREMAN-

Commercial hydraulic motors bearings/shafts are not intended to handle radial loads, they are only designed to transmit shaft torque. Having said that, the primary consideration in the design of these shaft splines is to ensure the proper degree of constraint in the spline joint and motor mounting flange. A side fit spline allows for a small amount of radial misalignment between the motor shaft and its mating part. Root or major diameter fit splines are used where precise concentric location is required.

Good luck to you.
Terry

 

[Tmoose]

In addition to the motor shaft bearing radial load concerns by Terry, sheaves/pulleys/pillow block bearings secured to shaft by setscrews are very susceptible to loosening because the constantly reversing radial belt load WILL cause micromotions between shaft, hub etc which will slowly chew their way to freedom.

 

[ALREMAN]

Thanks for the help Terry and Tmoose. I will make sure our maintenance plan pays particular attention to the motor bearings and also that we have checks in place to ensure the drive sprocket is not coming loose. The motor I have selected does have a max side load at key rating so I believe the design of this particular motor does allow for some side loading. I am mostly trying to ensure that I design against catastrophic failure of the drive sprocket itself.