Spindle Deep-Dive

​To begin the design process for the headstock, I set out a number of detailed requirements below to narrow the design space. 

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• Operating temperature range 0C to 60C 

Influences bearing lubrication selection 

• Max operating speed >3000rpm 

Influences bearing selection and lubrication 

• Designed continuous operating loads: Axial 100N, Radial 200N 

Influences bearing sizing 

• TIR at spindle nose <5 tenths 

 Adds machining complexity.  Measured at <3 tenths 

• Axial unloaded play <5 thousandths 

Related to axial preload.  Measured at < 1 tenth 

• Radial Deflection 10N preload to 200N load <5 thousandths 

Influenced by bearing seals 

• Axial Deflection 10N preload 100N load <5 thousandths 

• Vibration magnitude from low speed to high speed will increase no more than 50% as measured with a phone accelerometer. 

• Expected bearing lifetime and service interval >10,000hr 

• Must be totally rebuildable and require no intermediate maintenance 

• Bearings must be shielded from dust and debris 

• Power loss through the spindle <20% of motor output.   

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After creating these requirements, I proceeded to do a deep dive into bearing design and how they come together to support spindles.  I read through the Timken bearing manual, as well as going through the documentation on the SKF website.  There is so much interesting information out there!  In learning more about bearings and how they function, I came across the field of Tribology, and have started reading Introduction to Tribology by Bharat Bhushan which contains a ton of information some of which I even applied in the bearing selection process! The video series by Robin Renzetti regarding spindle design and rebuild also proved very helpful. With all this information about bearings, and how to situate them to support spindles, I was ready to move onto the next step of actually choosing bearing types and sizes, and their positions.  This was the point where I ran into another implicit requirement, cost!  Matched angular contact bearings are very expensive so my plans of using a set at both ends of the spindle were infeasible.  Next, I planned on using tapered roller bearings, but my requirement of shielding and long service interval didn’t exclude open bearing designs completely but would have made them significantly harder to implement.  Since the spindle doesn’t have to operate at particularly high speeds, the lubrication requirements are such that a permanently sealed bearing would suffice.  In the end I concluded that using a double row sealed angular contact bearing (SKF 3205A-2RS1TN9/MT33) to support the radial and axial loads at the spindle nose, and a sealed deep grove ball bearing (SKF 6304 2RSJEM) at the outboard side to principally support the radial loads from the belt drive.  This design decision was a compromise to minimize the complexity of machining as well as staying in budget. 

 

Next came the analysis piece.  SKF has a super helpful tool, SKF Bearing Select which allows you to specify bearings, their center distance, and a load case, and will output all the data out could want including rated life, static and dynamic factors of safety, and even an estimation of frictional losses through the system.  The way I set up the CAD enabled me to vary the center-to-center distance of the bearings parametrically and thus easily adjust this critical part of the design.  

 

Report from the SKF analysis tool