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[joppyuk1]

I've just fitted a motor into a model, and because the motor is higher than the prop shaft I've had to use pulleys. The one on the shaft has a grub screw to hold it fast, but the one on the motor is a push fit. This one comes off after a few seconds running!

My thought is that either a) it's too loose or b) the pulleys are not quite correctly aligned and the torque(?) means the band from the lower (shaft) pulley is slowly pulling the one of the motor off.

Can I a) put a dab of glue onto the upper pulley to hold it in place or b) try moving the lower pulley so that it pulls the upper one the other way?

I know there's probably a simple explanation and solution, and you all know it, so "help"!!

ij

ps good job it's happening in the pool in the garden and not out on a pond.

[Colin Bishop]

It's probably too loose but possibly the belt might be a bit tight. Ideally you need to be able to adjust the motor fractionally up or down to get the optimum tension. The best way to measure this is with an ammeter and a low voltage battery which just keeps the motor ticking over. Then adjust the tension until you get max speed/lowest current draw.

The push fit pulleys can be a problem as they are usually made from nylon or 'slippery' plastic which doesn't take glue very well. Once they start slipping it is hard to rectify. Also, if you glue the pulley to the shaft then the only way to remove the shaft for maintenance without trying to get the pulley off again is to withdraw it into the boat and there isn't always room. I have exactly the same issue with my plastic magic kit at the moment but I do have room to withdraw the shafts into the boat. As the shafts are only 2mm with 20mm props I will use a bit of epoxy to help secure the pulleys to the shafts. If your pulleys have central extensions it is sometimes possible to drill a small hole into these and use a small cheese head bolt, say M2 or so to self tap into the hole and act as a grubscrew. Obviously the hole needs to be fractionally smaller than the bolt! It's a fiddly solution but it does work!

Colin

[Captain Povey]

Hi, In this instance I would always use a grub screw to fix a pulley to a shaft, especially a motor shaft which can get hot and cause the the plastic to 'relax' its grip on the shaft.  Having said that I spent 39 years working for a company, designing products that relied on plastic discs being pressed on to small motor shafts often rotating at speeds up to 20,00rpm. Generally the fit was so tight that the disc removal could sometimes pull the shaft out through the motor laminations. Anything looser invariably meant after some time the disc would fall off. Graham

[Captain Povey]

Hello again, Sorry, that should be 20,000rpm, typing ws never a strng point. I also forgot to say that I often grind, with a dremel or similar tool, a small flat on the motor shaft for the grub screw to sit down on as the motor shafts are very smooth and hard preventing the screw from biting into the shaft. The high quality finish on the shaft, which ensures the plain bush bearing don't wear to too quickly, also prevents an adhesive working well so if you do have to resort to glue I would roughen the shaft first taking care not to let the dust get into the motor. Graham.

[joppyuk1]

Thank you for the suggestions. I pinched a grub screw from an old electric switch, drilled a hole and fitted it (quite a job with me having five thumbs on each hand, and working with small bits of metal that disappear onto the floor every two minutes) and it seems to have done the job.
The moral of the story, as far as I'm concerned, is to use a direct drive wherever possible. Lesson learned for the next build.

ij

[Colin Bishop]

The moral of the story, as far as I'm concerned, is to use a direct drive wherever possible. Lesson learned for the next build.

Not necessarily! Pulley drives have a lot going for them in small and medium sized models and larger ones too if toothed belts are used.

They are quiet
They offer more options in positioning the motor (as you have discovered)
They offer the opportunity to easily gear down the motor which is almost always a good thing.
If the prop gets jammed then the belt will quite likely slip or jump off limiting any damage to the electrics.

Most of my boats have pulley drives for the above reasons but of course you do need to be able to firmly secure the pulleys to the shafts. RipMax used to sell a very good selection of pulleys with brass centres and hardened grubscrews but alas they are no longer available. Removing the plastic pulley from the brass hub also provided an excellent basis for making up a custom tiller arm for the rudder.

Colin

[malcolmfrary]

I found these guys - http://www.motionco.co.uk/timing-pulleys-timing-pulleys-plastic-c-25_35_26.html - while looking for ideas using toothed belts.  Unfortunately their smallest bore is larger than the shaft diameter of the small motor I was intending to use, so I would have to figure an adapter.  On the plus side, they do have an on site calculator to correlate any combination of pulley teeth and spacing.


For the tiller arm, I have found a source of brass arms with a nicely machined hole and threaded for a locking screw, also brass, which only needs a hole drilling to accept the steering link, but might need filing a bit thinner if a clevis is to be used.  The shop calls the package it comes in a "13 Amp plug", but we know better.

[Colin Bishop]

These people also do nice pulleys but they are a bit expensive! http://www.technobotsonline.com/pulleys-and-cams/aluminium-pulleys.html

I have asked Mark of Marks's Model Bits whether this is something he could provide (once he has got his new premises sorted out).

Machining the whole thing out of the solid would be a bit of a job but I am wondering if the pulley and the hub could be made separately and then welded or Araldited together which should simplify things and was really how the old RipMax pulleys and gears were made except that the outer part was plastic/nylon.

Colin

[joppyuk1]

Having built to completion my first model boat since schooldays (quite a few abortive attempts) and just retired, I find I have a lot to relearn, especially as my recollection of suppliers dates from the 1960s. I'm currently looking at a selection of plans - all those free plans from Model Boats over the years just pile up, plus the others I've purchased through the years - mean I'm spoilt for choice. The electrical side is total magic to me, and another language entirely. That's why the forum is so useful to cack-handed folk like me. Someone (or two, or three) can always come up with an answer. Just wait till I start asking about radio control1