I’m looking at making helical gears, as per normal I wanted to start with the fundamentals. What is the optimum angle? Can I index them on the milling machine? Can I machine with a normal cutter?
Why do I need to make helical gears? The big benefit is they will be quieter. Gear noise on my double acting oscillating engine is an issue as the forces change direction. How much quieter could this engine be with a set of helical gears?
I could go and buy a pair of helical gears from HPC Gears.
There would be a number of advantages though. I would get a matched pair. The angles would be optimal. They would look stunning.
However, where would the fun be in that?
So, back to making helical gears.
Optimum Helix Angle
Helix angles vary from 5° to 45°. Single-helical angles fall between 5° and 20°, and double-helical angles fall between 20° and 45°.
The maximum efficiency is a helix angle between 40 and 45 degrees, however a reasonable efficiency is achieved above 15°. Due to difficulties in forming the thread, helix angles greater than 30° are rarely used.
An optimisation of the transmission error versus helix angle shows minimums at 25° and 46° [reference 2]. However, both of these angles might be very difficult to achieve with an indexed cutter on a small general purpose milling machine.
The HPC Gears have a helical angle of 17°45′.
For a single helical the thrust force increases with helix angle. This means a thrust bearing is required and careful design of the gear is needed as it will see a bending force.
The noise produced by a single helical gear is lower than the spur gear and lower than a double helical [reference 3]. This paper also shows that the noise level decreases as the helix angle increases, although this is only shown for HA 27° and 34°.
The efficiency of the gear pair drops off significantly below 15° helix angle.
This angle could be achieved by placing the rotary table on a wedge machined at an angle of somewhere between 5° and 20°. As the angle increases I will have to decide if the depth of cut is positioned to be correct in the centre or at the edges of the gear face. Correct at the centre will leave the gears maybe not even meshing at the edges. A shallower angle will not create such a pronounced problem.
For my first attempt at making a pair of helical gears I think I’m going to start at a helix angle of 10°. This feels quite low on all counts, but I think this is achievable with the machining setup I have. Also, I feel confident that this will give me a gear pair that works.
References
- Meherwan P. Boyce, in Gas Turbine Engineering Handbook (Fourth Edition), 2012
- J S Kang, Y-S Choi, “Optimization of helix angle for helical gear system“, Journal of Mechanical Science and Technology 22 (2008) 2393~2402
- F. B. Oswald et al, “Influence of Gear Design on Gearbox Radiated Noise“, Gear Technology