Connecting Shafts of 2 Very Different Diameters

[Steven123456789]

I want to ask, if I want to connect a shaft from a motor that only has a diameter of 4 mm to a steel rod with a diameter of 30 mm, what should I do so that the motor can rotate the steel rod?

 

[Tom.G]

Either a machine shop or belt and pulleys.
(I suppose if you need very low torque you could use bead chain instead of a belt, but the appropriate pulleys are somewhat scarce.)

 

[Rive]

a motor that only has a diameter of 4 mm to a steel rod with a diameter of 30 mm

What that enormous size mismatch is telling me, is that likely there are other under- or overdesigns there somewhere. Could you please share more about that ... machinery?

 

[Baluncore]

Welcome to PF.

How you couple the shafts will depend on how the 30 mm diameter bar is supported, on a frame by one or more bearings, of what type?

Is it balanced?
What is the maximum operating rpm of the motor and bar?

How is the motor mounted to the frame?
Can it self-align with the 30 mm bar?
Or must the coupling be flexible?

How much torque will the motor be providing?
Or what is the 30 mm bar being used for?

 

[Steven123456789]

Welcome to PF.

How you couple the shafts will depend on how the 30 mm diameter bar is supported, on a frame by one or more bearings, of what type?

Is it balanced?
What is the maximum operating rpm of the motor and bar?

How is the motor mounted to the frame?
Can it self-align with the 30 mm bar?
Or must the coupling be flexible?

How much torque will the motor be providing?
Or what is the 30 mm bar being used for?

2 bearings, ID 30 mm x OD 37 mm x width 4 mm
Torque the motor providing - 0.4 N cm
the 30 mm bar will be used to rotate the mechanism

 

[Baluncore]

What is the maximum RPM?

 

[Nik_2213]

If low torque, classic trick is to spring-mount motor with small rubber wheel directly against friction band on cylinder. Like electric-assist bikes did before hub motors....

Plan_B is to machine end of bar down to compatible size, connect motor shaft via 'universal' or flexible joint.

If only needs to rotate in one direction, perhaps centre-tap bar, saw the head off compatible part-threaded bolt and connect that to motor shaft via 'universal' or flexible joint.