Reworking a Spider/jaw coupling

[duncan webster 1Participant @duncanwebster1]

To introduce some drift into this topic, you could turn the engine sideways and use a belt drive. Not ideal as it would impose a sideload on the engine, which it might or might not like.

[Charles LamontParticipant @charleslamont71117]

I have made my own coupling for connecting my Westbury Seagull to its starter. It is a double coupling and is 9/16″ OD. It incorporates a belt drive for the water pump. The parts of the mould used to cast the polyurethane rubber spiders are at the top of the photo. The first ones I made were too soft. 70 Shore was about right.

[martin haysomParticipant @martinhaysom48469]

can get lock nut’s and bolts with smaller heads. most [not all] have 12 point heads. i use them at work. dont have a name for them so i am not much help in finding them .but there must be lots of them on that well known online auction site

[Howard LewisParticipant @howardlewis46836]

Being bone idle; the thought came to me of buying a coupling with a 10 mm bore, and bushing one side down to 8 mm (Might mean opening up that half to 12 mm, and bushing down from 12 mm to 8mm to get a reasonable wll thickness, with added Loctite for security.

The flywheel could be counterbored to fit as Jason suggests, and with a suitable slot on one side used for a cord for starting.

Foe maximum flywheel effect, the OD of the flywheel should be as large as possible (As long as you do exceed the maximum permissible overhung load  ( To avoid crankshaft breakage due to fatigue – I’ve known it happen, at least twice!)

Howard

[Paul McDonoughParticipant @paulmcdonough43628]

Agreed Duncan, I have previously used belt drives to great effect, easy to align, durable, quiet and allows for easy ratio change, in this case the engine and shaft a just a little too long to fit across the width of my loco.

[Paul McDonoughParticipant @paulmcdonough43628]

Nice work Charles, I may yet end up making the part attached to the engine and keep an eye open for a coupling that can donate the motor end and rubber spider

[Paul McDonoughParticipant @paulmcdonough43628]

I like your thinking along the tines of Jason’s proposal, it’s not so much being bone idle it’s that these things are available at very reasonable prices, are proven and can be adapted to suit a specific requirement.

I appreciate your point about large flywheels I’m always tempted to go just a little bit bigger for smoother running.

agreed about the groove for the starter cord, in my case I’ll loop a longish belt on before coupling up, that way I can use a model boat remote electric starter if needed, although I am still lured by the idea of a built in electric starter it feels a little decadent.  :0)

[Paul McDonoughParticipant @paulmcdonough43628]

Thanks for the tip Martin, that sounds like an excellent idea

[Paul McDonoughParticipant @paulmcdonough43628]

Thanks everyone some great ideas to get me thinking.

i ordered the materials for the ‘generator’ sub frame which should cradle on rubber isolation bushings between the main loco chassis rails, dropping the unit lower down.

im looking forward to building this sub assembly.

[JasonBModerator @jasonb]

Looking at a few images of the Stinger engines it looks like the prop driver may have a grun screw to retain it. If so that should make it easy to replace unlike ones that have the “D” shaped hole. In that case you would not need such a deep recessed “cup” shaped flywheel.

I don’t think a particularly large flywheel would be needed. From my days of running 1/8th scale buggies there was very little in the way of flywheels, just the combined clutch and knurled ring for the friction starter and they were usually aluminium. A piece of 50mm steel should be quite adequate and being directly coupled to the motor once that is spinning it will have some flywheel effect.

[Robert Atkinson 2Participant @robertatkinson2]

If you use an “outrunner” style BLDC motor as the generator / starter it will provide more than enough flywheel effect.

[duncan webster 1Participant @duncanwebster1]

Driving a flywheel via the coupling will give the coupling a hard time. Is it possible to mount the rotor of the generator overhung direct on the engine crankshaft, then no coupling required

[duncan webster 1]

On duncan webster 1 Said:

Driving a flywheel via the coupling will give the coupling a hard time. Is it possible to mount the rotor of the generator overhung direct on the engine crankshaft, then no coupling required

i once had a Honda powered generator that was built that way

[Robert Atkinson 2Participant @robertatkinson2]

It will considerably reduce stresses on the crankshaft which is a lot more expensive and harder to replace than a new coupling resillient insert. It will act is a simillar fashon to the inertial damping pulleys and flywheels found on a lot of modern car engines.

Robert.

[Paul McDonoughParticipant @paulmcdonough43628]

Interesting points, In the past I have built direct coupled rear axles for tether cars, in that case I had no flywheel just a solid coupling, pinion and crown gear and the rear wheels, but for this build convention lead me to use one anyway. The outrunner motor despite having significant inertia is quite a draggy load.

if you spend some effort to align the parts a solid coupling is viable.

i quite like the idea of adding the flywheel element to a suitable coupling

[JasonBModerator @jasonb]

I’m not sure if cost is a major issue with this project. £250 approx for the engine shown when you could get a new weed whacker engine for 1/4 of that which includes a fuel tank and recoil start.

[duncan webster 1Participant @duncanwebster1]

The engine is designed to have a fairly big propellor rigidly attached, I reckon that would have more rotating inertia than a relatively small flywheel, so the crankshaft should be OK as far as any torsion effect

[Paul McDonoughParticipant @paulmcdonough43628]

Hi Jason, I have considered a weed-wacker engine but the lack of dimensional info put me off, I also considered stripping back a more civilised Honda GX25, but common to these types of engine they are a bit too bulky, ventilation and magneto parts seem to stick out in all the wrong directions. At least I am confident that the RCGF20cc can be made to fit in the chassis.

as the members of our model engineering club have said several times, why have you made it so small!

[Paul McDonoughParticipant @paulmcdonough43628]

I guess I could do some sums and chech this.

i am quite tempted to give the engine a bit of running in on a prop

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