My children gave me a 6’ replica Sopwith propeller for Fathers
day and by a happy coincidence we needed a ceiling fan for the conservatory. A wood burner heats the conservatory in the winter but the air could really do with circulating.
Never having designed anything like this before it took me a while to work out how to do it. Bolting the propeller onto an existing fan hub probably wouldn’t be safe (given the weight of the thing) and certainly wouldn’t be any fun. I eventually settled on a bit of a Steampunk theme with an externally mounted motor and a chain driven 3 speed gear system.
Required parts if you fancy having a go:
- 6’ propeller, scaffold pole, M18 studding with nuts and washers, taper roller bearing.
- 4 x 1275cc A series conrods.
- From a bicycle a Sturmey Archer hub gear, front wheel hub, rear cassette, chain and jockey wheel.
- Counter balance weights, three appropriately sized pulleys, wiper motor, motor power supply.
- Christmas pudding pot.
- A selection of fixings and other bits’n’bobs.
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The propeller is hung on a length of case hardened M18 studding suspended from a tapered bearing fixed in the end of a scaffold pole. The use of only one bearing allows for any rotating imbalances in the system. A couple of rubber washers adds a little damping.
A pair of bearings (one above the other) would have made life easier but any slight imbalance would have resulted in flexing of the portion of the studding between the lower bearing and the propeller. I was keen to avoid this so the single tapered bearing was used which allows the inner race to simply lift from the outer race if the studding is not exactly perpendicular. Ultimately, if the fan were run unbalanced for significant periods of time the bearing would wear unevenly but at least it is safe.
Tapered bearing outer shell sits on stainless screws in scaffold pole.
Three speed Sturmey Archer gear hub. This is mounted vertically from one of the conrods and is suspended from one end only. The gear lever for the hub is mounted remotely on the wall.
The fan could have had an electronic speed control, but gears are more fun.
Early designs used a chain drive between the gear hub and the propeller cog and was quite simple but this soon proved to be unsatisfactory. The chain was replaced with a rubber belt drive which allows for any slight play in the system.
Who would have thought that Austin Mini 1275cc A+ connecting rods are a perfect fit for scaffold poles?
Four rods are needed to allow for counterbalances and the other components. Using the conrods allows infinite angle and height adjustments.
Early trials. Because the propeller hangs from a single bearing and is free to float the tension on the drive belt from the gear hub pulls the assembly to one side. I didn’t think this would be a problem but the fan proved to be quite sensitive to side loads. Some method was needed to pull the assembly back into the centre, the tension system on the left was fitted and uses a bicycle front wheel hub.
The blue motor proved to have a noisy gearbox and was later swapped for a 24V windscreen wiper motor.
The whole fan assembly hangs from a single rubber isolated hook in the ceiling. With the heavy motor and gear hub to one side of the centre line counter balance weights are required to bring the assembly back to vertical. The weights are a combination of odd pieces of steel I had lying around and two solid brass curtain pole ends.
The fins are simply there for effect.
The Christmas pudding pot, once painted and fitted it hides the ceiling hook.
Rather nice ebay find for the control box.
The bicycle gear lever is mounted in a wall mounted control box along with a couple of switches and the ammeter.
When riding a bike gear changes are much easier if the cyclist reduces pedal pressure a little and the same is true for the fan. To replicate this action for the fan there is a non-latching power interrupt switch which is pressed while changing gear (button the right). This makes changing gear a two handed, but curiously satisfying operation.
The ammeter displays the current which changes with gear selection and for night time operation I managed to squeeze an LED into the display.
The convoluted tube is a length of vacuum tube from work and hides the electrical and gear change cables.
The three speed gear hub gives approximately 60, 90 and 120 rpm.
The propeller is a display item and is not balanced, either aerodynamically or by weight. Because the propeller and bearing essentially float any imbalances in mass or lift while rotating results in a knocking noise as the inner bearing race lifts slightly from the outer race at certain points in the cycle.
It took quite a while playing with balance weights and aerodynamic flaps to eliminate this. Invariably the combination of weights and flaps would work for one gear setting or perhaps two, but never all three gears. Also, the two bands (hub gear drive and tensioner) pull against each other. If they are not precise lengths the propeller is pulled slightly to one side and this adds another variable.
The fan was installed for Christmas 2014 and has proven pretty reliable and certainly moves the air around the room. particularly in 3rd.
A quick update.
As the propeller is suspended on a single taper roller bearing it is free to wobble if the system is not perfectly balanced. Initially this seemed a sensible idea as it could impart no bending forces on the studding used as the axle. However, in practice the prop has proven to be very sensitive to drive belt and tension belt lengths and also requires accurate balancing. Under certain conditions the prop wobbles and lifts the upper section of the taper bearing resulting in a knocking noise, disconcerting for those who didn't know what was happening.
As the whole assembly hangs from the ceiling on a rubber mount and is free to move I decided that a second bearing could be installed without too much risk of the axle flexing. This second bearing is fitted at the top of the axle to reduce the wobbling and has eliminated the knocking. The bearing is sleeved in rubber to allow a little movement, a piece of bicycle inner tube served the purpose and satisfied my inner Womble.
The pdf shows the final configuration for anyone who might be interested in the detail.