Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[GG]

Part 1
A modeler can quite often find a problem in matching a motor of the right power for a model with the size of propeller that is wanted to be driven.  Usually the motor turns too fast and would be grossly overloaded if connected to the prop. First thoughts might turn to the use of gears or possibly a toothed belt but, there is however an alternative that might be just as good if not easier to use.  This is the humble and maybe overlooked pulley system.


Just in case anyone is not sure what this is, the basic set up is shown in Fig 1.  Both motor and prop shafts are fitted with pulleys which have a grove around their circumference.  A belt, band or call it whatever you like, fits into the groves and connects both pulleys.  When the motor shaft turns, friction between the pulleys and belt cause the both pulleys to turn.


If both pulleys were identical then you would expect both to rotate at the same speed.  This might actually be useful where the motor speed is OK but it is impractical to get the two shafts in line.  However, if we wanted to reduce the motors speed then the pulley on the prop shaft has to be larger than that on the motor shaft.


The actual speed reduction is the number of times the motor pulley must turn to make the prop shaft make one turn.  This depends on circumference of the pulleys which is proportional to their diameters (remember Pi x Diameter) so the speed ratio of any pulley system in a model boat is simply;


        Diameter of Prop Pulley  :  Diameter of Motor Pulley


The "picky" amongst you might want to question "which pulley diameters" we ought to measure.  Is the the overall diameter, the diameter at the bottom of the grove or somewhere in-between?  In practical terms I doubt it matters that much in our models as long as you use consistent measurements.


But, before going any further it has to be said that pulleys, like gears, can not magically increase the power of a motor.  They are still limited by the simple relationship;


             Power = Torque x Motor Speed


They can allow both motor and propeller to work more effectively which ought to be worthwhile.
Glynn Guest

[warspite]

In belt driven fans, say used for moving air down a duct work system, it is usual to have a minimum of two belts and in most cases they usually have 3+, though in models they are vastly smaller and dont have as much of a problem as say a 3000rpm motor at 15kw (and I am not talking about brushless either)  .

[GG]

Part 2
Friction is often regarded as a bad thing in mechanisms and might tempt people to think of lubrication.  In fact in a pulley system friction is essential if the belt to to transmit power between motor and prop shaft.  Any slippage on either or both pulleys will lose power and, likely as not, cause excessive wear on the belt.


This can lead modelers to putting the belt under a large tension force, in other words stretching it tightly over the pulleys.  This might make slippage next to impossible but it does apply large side loads on both motor and prop tube bearings, bad for friction and wear, plus it increases the chance of the belt breaking.


What is needed is just enough tension in the belt to avoid it slipping under normal conditions.  This can be achieved by testing the model in the water and adjusting the separation between the motor and prop shafts, Fig 2.


If the motor cannot be moved then an adjustable "Idler Pulley" is an option.  It's also worth pointing out that the motor does not have to be above the prop shaft, it can lay alongside.  I've sometimes had the motor in this position to save space inside the hull.


One thing to watch is that when transmitting power, one side of the belt is under increased tension whilst the other side has decreased tension, Fig 3.  This is the way power is transmitted between the pulleys and you may be able to see this effect as one side of the belt becomes looser and may vibrate more.  But, taken to extremes it might allow the belt to jump off the pulley.  If this happens and the pulleys are properly aligned, then increasing the static tension of the belt is usually called for.
Glynn Guest

[GG]

Part 3
It used to be possible to visit a local hobby shop an buy suitable pulleys but now you probably need to carry out an internet search.  Luckily, it is no great chore to type into a search engine some combination of "pulley", "model" and such like to quickly locate potential suppliers.


Metal and plastic pulleys can be mixed with no problems.  I would suggest buying those which are secured to the shafts with grub screws, along with suitable "flats" on the shafts.  The plastic "push-fit" pulleys can work at modest power levels but depend on the right combination of hole and shaft sizes to work.  An alternative could be a pulley with a plain hole that is a close fit on the threaded end of a shaft and secure it with two nuts, one on either side.  I have had success with this method.


As for the drive belts, the humble elastic band can be used but needs care.  The correct size (both length and cross-section) is needed to reliably transmit the power.  They also have the habit of deteriorating over time and losing tension if not actually snapping.


I have found "O rings", often used for sealing joints and connections, to be much better.  They are tougher and with a round cross-section seem to create a grip in the pulley groves.  Mine used to come from the plumbing section of hardware stores but a few years ago a box of assorted sized was bought and will probably outlast me.


Glynn Guest 

[canabus]

Hi GG
The main problem is finding the specs on the motors at a voltage, but after a long crawl around the old interweb I found some answers !!!
Mainly for 540 rock crawler brush motors.
These are on 7.2 volts unloaded rpm:-
20 turn 18400
27 turn 15000
35 turn 12500
45 turn 9500
55 turn 7700
65 turn 7000
80 turn 5500

The 55 and 65 turn will happily turn a 3 or 4 blade 50 to 60mm prop in fishing and tug boats.

If you wish to go to brushless motor divide the rpm into the voltage to get the kv.
Brushless numbering is generally dia., length and kv.
eg changing a 80 turn brush motor, 5500 rpm divide by 7.2 equals about 750KV.
So a 3639-750Kv is the same bolt pattern as a 540, but shaft size is generally larger(5mm).

Running down the above motor to brushless will be about 2500, 2000, 1800,1250, 1000, 950 and 750KV.

Matching to you prop, I worked on the Raboesch prop chart maximum rpm.

It's a basic start.

Hope this helps someone on the forum.

Beers and Cheers
Canabus

[GG]

Part4
Pulleys can be a neat way to drive twin props from a single motor, Fig 4.  One problem might be the reduced contact between the motor pulley and belt. leading to slippage.  If the motor cannot be raised high enough to increase the tension sufficiently, then the the use of a double pulley on the motor shaft could be the answer, Fig 5.


If opposite rotation is needed in a twin prop installation than the double pulley on the motor shaft can also be used.  Now one of the belts is crossed over to reverse the rotation of one prop shaft, Fig 6.  Some care is needed to ensure that the belt does not rub against itself where it crosses over to avoid friction and wear.


Glynn Guest. 

[GG]

Part 5.
Pulleys can not only be used to alter the speed of rotation of a shaft, they can also change the direction of the axis of rotation. A obvious example is when the output shaft axis is perpendicular to the motor shaft but a couple of pulleys and a belt can still connect them, Fig 7.


If the output shaft is some distance away from the motor then a couple of "Idler Pulleys" plus a longer belt, might do the trick, Fig 8.  Some care is needed to get the belt tension just right, not to slack to risk coming off the pulleys but tight enough to cope with the torque.


Glynn Guest.

[GG]

Part 6
Rotating items on a scale type of model, such as the turrets on a warship, can be achieved by using a servo.  The only problem is that the standard servos have a total travel of some 90 degrees which might not be sufficient for the desired effect.  Yes, you can get servos that rotate through 180 degrees and I have used them but what if you want more?


It is possible to increase the servo travel in some RC outfits but this demands reading and understanding the instructions. Another way could be to buy and extra piece of "add-on" electronics but even then problems can arise when the servo mechanism hits the "stops" and makes a loud and painful buzzing noise.


You could then open up the servo and attempt to make alterations from which there may be no coming back from if unsuccessful. A safer method might be to use the pulley system to get the increased rotation at less risk, well I can't actually think of any risk?


Take the rotation of a gun turret on a warship, it may have a total rotation of something like 270 degrees.  Any greater and there could be the risk of the barrels hitting the superstructure or, worse still, firing shells through it.  If we take a pulley system with a ratio of 3:1 and fit the large pulley on the servo and then the smaller one under the turret, the 90 degrees of the servo will be increased to 270 degrees movement of the turret, Fig 9.

[/size]In fact you don't really need a pulley on the servo, a suitably large servo arm will do.  The same effect can be produced by tying a cord to one end of the arm, then wrapping it around the turret pulley a few times and finally tying it off at the other end servo arm.  To maintain tension, a small spring can be fitted into the cord but only where it will not foul the operation.There are many ways to transmit power between two rotating shafts that are not in line.  It is important that the method used has the qualities of smooth low friction operation with little chance of failure.  Pulleys might lack the aura of precision and accuracy that gears appear to have and may not be suitable for all possible applications.  But, used with care, they can work with quiet efficiency in our models.Glynn Guest.[size=78%]

[RST]

Didn't Colin or Dave or somebody issue something similar recently here or on modelboats forum? Trouble is, I think give this a month, nobody will look at it or even think of it on a search.  It's all good basic info combined but how do you get folk to read it nowadays?  It's not an instant answer to a specific question which seems key to be modern life for many these days.

[Capt Podge]

For me, assuming I'm not breaking any forum rules, I shall copy these diagrams to my hard drive for future reference. You never know whether or not you might need one or more for a future build. 


Regards,
Ray.

[GG]

RST   The problem with "instant answer to a specific question" is that whilst it may solve the immediate difficulty, it rarely generates a proper understanding of what went wrong.  The poor soul then lurches on to their next problem and asks for another "instant answer" and so the cycle continues.


Having said that, some people seem perfectly happy to work that way.


I posted this item in the hope that it would make a primer for anyone who has never used them but might be able to solve a problem with suitable pulley/belt system. 


Perhaps the forum could benefit from a section where reference material could be placed?
Thanks for the comments, Glynn 

[Dreadnought]

This is really interesting thanks GG     

[GG]

Canabus.    I started using "rock crawler" motors several years ago (Goliath aircraft carrier Model Boats Sept 2013)  and rewinding tired 540 motors with something like 100 turns even earlier.  I've never regretted using them in my models.


But, even the humble 27 turn 540 motor can be tamed by using a lower voltage battery.  The river ferry Ogdensburg (Model Boats June 2013) used such a motor with a 2 Volt supply and worked perfectly.


Maybe this is all something else that could be put in a Mayhem reference section?
Thanks, Glynn

[kinmel]

Once upon a time this was relevant, but is becoming less so every year as we learn to embrace the much greater torque provided by brushless motors and the variable power curves of modern radio equipment.

The formula Power = Torque x Motor Speed will never change, but motor technology has and the inefficiencies of a brushed motor using gears or pulleys simply cannot compete with a suitable direct drive brushless power line.   The high capacity and power output of LiPo battery technology makes the comparison even wider.
These simpler, more effective technologies don't need a 6 part explanation, they have a much gentler learning curve that suits the modern approach to problems.

[Colin Bishop]

The brushless motor I have in my SLEC Huntsman certainly delivers on the power but the incremental speed changes at low revs are rather lacking. It's a bit of of half power and upward from there which is not necessarily what you want with a scale boat.

Not sure if it is the motor or ESC that is the reason but brushed motors and conventionsl ESCs do seem to offer better slow speed control.

Colin

[kinmel]

You need to update your R/C as well as your power line technology 
Control at slow speed is readily achieved with modern transmitters that allow you to vary the channel's output curve.

[Colin Bishop]

Maybe so but I don't necessarily want an expensive TX.

Most of my models are conventional scale and a traditional combination of NiMH batteries and standard R/C works fine with more than adequate run time plus slow control. Cheaper too! (and I don't have to keep the batteries in a fireproof wallet!)

Sometimes less is more.

Colin

[RST]

It's a great write up!  Can I find that article now, -can I flip!

Re brushless etc.  Yes but it doesn't address one point which pulleys or gears address if you really have no choice.  Actually I was looking at this myself the last week, but managed to get better access straight-off and adjusted a few things to fit a motor direct drive.  And I agree with Colin, for my stuff, no need for brushless when cheap old technoogy works just fine (I don't buy a new car, TV, phone or laptop just because a new one is out but I guess allot of folk do).

For what it's worth before E-Bay, these guys used to be a good source of pulleys, cogs and gears.  A long time since I last ordered and I'll bet they don't give the little books away for free now.  Some smart *ss will always say you can buy from China cheaper now as well but for anyone looking for this kind of stuff...

https://www.hpcgears.com

(edit -can't see pulleys now but hey-ho)

Rich

[Colin Bishop]

As always, horses for courses.

Colin

[red181]

https://www.mfacomodrills.com/pdfs/belts%20&%20pulleys.pdf


this is where I get my belts and pulleys. I've done this in two boats now, one has since moved on to a new owner, one stays with me. I like the tinkering more than the sailing, and by using different ratio's, when I was sailing the boat often, I could easily change the ratio to suit the lake.


Big lake, lower acceleration but high top speed, small lake  high acceleration and lower top speed. I used on board data logging to analyse amp draw, gps speed, battery life and so on, to achieve the optimum performance. Sometimes what "looks faster" actually isn't, and combine this with the size of the prop.


I gave the testing info to Simon at propshop, and he was able to make me a cleaver 3 blade prop to suit the results, I was testing with cheap 2 blade "x" props.


Back to the gears, My pal made me a custom motor mount which allows adjustable tension. During the testing it became apparent that I needed a supported the end of the shaft as it was "whipping" and after a few runs was bending, probably due to side loading as mentioned earlier.

[GG]

Red181,
      Neat installation and a good point about supporting the top of the prop shaft especially with high powers being transmitted.
Which reminds me that it may be worth describing how I build geared and pulley drives into models.
Glynn
[size=78%] [/size]

[frogman3]

https://www.mfacomodrills.com/pdfs/belts%20&%20pulleys.pdf


this is where I get my belts and pulleys. I've done this in two boats now, one has since moved on to a new owner, one stays with me. I like the tinkering more than the sailing, and by using different ratio's, when I was sailing the boat often, I could easily change the ratio to suit the lake.


Big lake, lower acceleration but high top speed, small lake  high acceleration and lower top speed. I used on board data logging to analyse amp draw, gps speed, battery life and so on, to achieve the optimum performance. Sometimes what "looks faster" actually isn't, and combine this with the size of the prop.


I gave the testing info to Simon at propshop, and he was able to make me a cleaver 3 blade prop to suit the results, I was testing with cheap 2 blade "x" props.


Back to the gears, My pal made me a custom motor mount which allows adjustable tension. During the testing it became apparent that I needed a supported the end of the shaft as it was "whipping" and after a few runs was bending, probably due to side loading as mentioned earlier.

   
HI RED181 do you know if MFA still sells the plastic cambelt drive motor mounting ?
chris

[john44]

Hi Chris, have you tried SHG model supplies.


John

[frogman3]

Hi Chris, have you tried SHG model supplies.


John

HI John no not yet as got to get the wife to phone up for me as I have a hearing problem an cant use the phone as to many yrs in a trailer makin factory welding an got hearing loss an the missus is not well  at the moment to phone up for me when she is better i'll ask her but I have down loaded their catolouge an  cant see any in it but many thanks to you for the suggestion cheers
chris

[red181]

As far,as I know that link to mfa still shows the belts and gears