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Flywheel Connection to Shaft Flange - Bolted Joint Analysis

Flywheel Connection to Shaft Flange - Bolted Joint Analysis

Flywheel Connection to Shaft Flange - Bolted Joint Analysis


I am currently designing a flywheel. The flywheel will connect to a shaft flange by using a concentrically locating shoulder on the flywheel that locates on the flange OD, 1 dowel pin that will be a slip fit to clock the flywheel and a number of bolts on a BCD to provide a clamping force between the flywheel face and the flange. The clamping force will transmit all the torque load.

Details as follows:
Clamping Bolts BCD = 80mm
Number of Bolts = 4
Design Torque = 300Nm (includes design factor of 5)
Ideal Bolt PreLoad Stress = 800MPa (Unbrako Catlogue)
Friction Coeff = 0.61 (aluminium to steel - clean and dry) (using 0.15 for greasy steel doesnt change selected bolt size)
Effective Friction Radius = 40mm (calculated based on eqn 9.42 Design of Machine Elements Textbook)

Minimum Required Bolt diameter = 2.4mm
Selected bolt Diameter = M5

Max Flywheel Speed = 7000rpm
Flywheel OD = 340mm
Mass - 4.5kg

I believe my calcs are correct but the 4 x M5 bolts seems very small. I've been looking at car engine flywheels which are a fairly similar size and mass and operate at a similar speed and they all have 6 x M10 bolts or something around that. Im worried that my calcs dont take into account the high flywheel speed. Am i overlooking an additional load? Are the engine flywheels connections accounting for vibration with a large design factor? I should also note that this flywheel will have a very constant torque load - no pulsing as its in an electric motor.

Any help would be much appeciated!


Alex - Junior Mech Engineer

RE: Flywheel Connection to Shaft Flange - Bolted Joint Analysis

Your bolts are small because your torque is small. 221 ft-lb isn't a lot. RPM does not matter- torque is torque. If it's truly constant (what's happening on startup? What's the load on the other end of the electric motor doing?) then you don't need a lot of capacity for a small load.

In a manual car transmission/engine design, the flywheel has to handle very very high transient loads; when the clutch is engaged, remember that the flywheel/clutch assembly transmits torque in both directions. Say you're in a manual trans vehicle that hits a pothole with a drive wheel; this results in an impact load to the drive wheel; the torque component of that impact load is transmitted up the drive train and all the components in that chain have to be sized for it.

If you truly have no possible transient or other worst-case loads above 300 Nm, you're not going to need big giant bolts.

With all that said, if I were in your shoes I'd probably still investigate using larger fasteners (at least M8), if for no other reason than to make the assembly much more durable across multiple assembly-disassembly cycles.

RE: Flywheel Connection to Shaft Flange - Bolted Joint Analysis

A car flywheel will see shock loads when the clutch is dumped.

I took apart a car once and couldn't get the flywheel bolts to come loose. 6 x 7/16-20 bolts. My impact was barely able to move them. I hit all the bolts with the impact an when the last one just started to move the flywheel bounced back and forth against the bolts about 4-5 times. It had slipped and was jammed up against the bolts so tightly it was badly binding them. They could be turned by hand after.

RE: Flywheel Connection to Shaft Flange - Bolted Joint Analysis

I don't see where the single bolt clamping force is mentioned.

RE: Flywheel Connection to Shaft Flange - Bolted Joint Analysis

Thanks for the informative replies SwinnyGG and LionelHutz, i will investigate shock loading for this application.

Tmoose I didnt include the entire calculation. But required clamping force from single bolt = torque/(friction radius*friction coeff*number of bolts) = 2130N

Thanks Guys

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