Engines and Gearboxes

[enginepeter]

You might have seen my earlier post concerning the lack of power to drive my heavy Black Prince and my questions asking for Datasheet information on the Taigen 390 motor.

Before reading any further I must make it clear I know little about DC motors and even less about the inter-reactions of Volts. Amps and Torque. No doubt someone more knowledgeable will have been able to make sense of the data sheets and arrive at a different conclusion!

What was I after – a high output motor at low revs. For a scale speed with my drive wheels maximum revs from the gears were 125 road, 90 off-road. I was going to stick with my standard 7.2v Nimh battery, not move on to higher voltages.

The 47mm body length of my present Taigen motors, assumed to be 390’s, were as long as I could fit in the space available without making custom gearboxes. I had already moved a motor on a 4:1 gearbox in order to fit the space available in my tank.

Space ruled out moving up to 480 or 500 series motors as well as my amps limit of 20a.

Research by searching RCTW indicated some praise for the Igarashi 5 pole motor and I then spent some time trying to get my head round the different datasheets of these and other motors. Some numbers were based on 12 volts, others on 7.2 volts.

I knew that a reduction in volts resulted in lower revs but the impact on amps and torque was lost on me. (I failed O level physics, twice!).

I first looked at the two gearboxes I had, Taigen 4:1 and 5:1. The gears in the 4:1 gave a ratio of 89 :1 and the 5:1 a ratio of 46:1. If I changed the motors on the 5:1 by re-mounting them, I could use a smaller pinion on the motor. Changing from a 24 tooth to a 10 tooth pinion would improve the gearbox ratio to 109:1. My thanks to Matdragon for his gearbox spreadsheet calculator.

The problem with gearboxes is the efficiency loss through the meshing of gears, brought about by a number of reasons, including poorly cut gears, badly fitted gears and the number of gear meshes. The number of gear meshes on the 5:1 are 6 compared to 5 for the 4:1 so with all other things being equal the 5:1 is going to have lower efficiency.

After purchase, my gearboxes had been run in for a while. Before doing any tests I stripped the gearboxes and cleaned them. I decided to do what I did on rifle triggers where any build-up of dust/dirt mixed with oil can cause problems. This was a trick given to me by a gunsmith many years ago. I soaked and brushed the gears with petrol in which I has mixed a few drops of 3 in 1 oil. When the petrol evaporates it leaves a thin film of oil on the gears. The gear pivots in the brass bushes were lightly oiled with neat 3 in 1, a small drop on the end of a miniature screwdriver.

I decided to test my two gearboxes by seeing what weight would move the gears, the motor being taken off. This was achieved by wrapping cord around the shaft and putting weight into the container suspended from it until the gears started to move. This was tried at different gear positions and identified a problem with my 4:1 gears, when a substantial amount of weight had to be added at one point. The meshing points of the various gears was marked with a spot of white paint, the gear stripped and examined under a magnifier. This identified a small spot of foreign matter stuck solid, that had not been removed when cleaning. This was removed with a sharp blade and only went to show that a minor issue could result in a major problem.

When the gears were working smoothly I found that the 5:1 needed 26% more weight to start moving rather than the 4:1. This experiment was not soundly based but this information was better than nothing and probably more indicative about gear friction than the general 10% factor quoted as the efficiency loss for each gear train mesh. This led me to think the advantage in changing the pinion to improve the 5:1 gear ratio over the 4:1 would be lost in view of the perceived efficiency loss in using the 5:1 gears.

The only improvement I could make to the 4:1 gearbox would be if I could find a 9 tooth or smaller pinion of the same 48 dp to match the present gears. This would give an improvement in the gear ratio from 89:1 to at least 99:1.

Next to look at motors.

Part 2 to follow....

[Jofaur86]

Good evening, we must beware of the definitions 3 / 1.4 / 1.5 / 1, it seems that does not correspond to a reduction ratio? attached a link for Tamiya
http://www.customrcmodels.com/Tanks/id78.htm

https://www.forum-rc-panzer.com/t7933-U ... tesses.htm
Désolé in French explication

[enginepeter]

Part 2

After looking at the gearboxes I then thought about conducting some experiments to see what motor suited me best. As mentioned earlier there was a lack of information about the standard Taigen Motors. I therefore decided to build a test rig in order to obtain some measurements.

My measurements of the Taigen motors would not be directly comparable to any of the data sheets available for other motors in view of the vagaries of my test rig. I then purchased other motors so I could compare results.

The test rig was made to measure the amps drawn, and provide an adjustable voltage supply. The addition of pulleys and a drive shaft running in ball bearings, enabled me to measure of weight lifted and stall amps.

Unfortunately, I did not have a 380 motor without fully stripping one of my older tanks.

I found that :

At 7.2 v No Load Speed (rpm) No Load Amps

Taigen 360 17,900 0.52

Taigen 390 23,900 1.65

MFA 385 LN 4,100 0.18

Igarashi 051-GC-5 8,600 0.27

Igarashi 048-GF-5 15,200 0.64

The revs were measured using a non-contact tachometer.

Next, I measured the maximum weight the motor could lift without stalling (just) and the amps used to lift the weight. These measurements were repeated to gain an average and there were pauses in the process so the measurements were not distorted by a hot engine.


At 7.2 v Weight lifted Amp Load

Taigen 360 69.0 grm 13.0 amp

Taigen 390 100.0 grm 23.0 amp

MFA 385 LN 92.0 grm 7.2 amp

Igarashi 051-GC-5 53.5 grm -

Igarashi 048-GF-5 98.0 grm 14.0 amp

I lost my note of the 051-GC-5 amp measurement and when realised did not want to set up the whole experiment again to repeat all the measurements.

It was clear from low weights moved that the Taigen 360 and the Igarashi 051 were not up to it for my tank and the MFA could be discounted as the RPM would be too low. The Taigen 390 maximum weight was exaggerated as over 20 amps were drawn and if reduced I guessed would be worse than the Igarashi 048.


The next experiment was to test the Taigen 390 and Igarashi 048 connected to the 4:1 gearbox.

Edit - The 4:1 gearbox was used for all experiments.

This test involved lifting a weight at maximum voltage – generally 7.4v with the amps also being measured. The weight were blocks of lead hence the odd numbers.

6.4 kg 8.3kg 10.6 kg 13.7 kg 15.5 kg

Taigen 390 8.4amp 11.0 amp 14.0 amp 18 amp > 20 amp

Igarashi 048 5.2 amp 6.5 amp 8.7 amp 12.0 amp 13.3 amp

One thing that amazed me was the sheer amount of weight the motor/gearbox could lift.

Finally, I tried to plot the weight lifted to rpm. This was not easy as I only had a short time span to measure, particularly for the lighter weight/higher revs. It was not easy with only one pair of hands to control the motor, hold the tachometer and keep an eye on when the weight is about to smash into the pulley. Numerous readings were taken to obtain average results that I felt comfortable with.

3.2 kg 5.45kg 8.15 kg

Taigen 390 216 rpm 192 rpm 148 rpm

Igarashi 048 140 rpm 127 rpm 105 rpm


The Igarashi 048 lifted the greatest weight, at the lowest amps and at a rpm that better matched my needs, so these were fitted to the Black Prince.

When re-assembled speed trials were conducted on a short pile carpet when the top speed on a moving start was measured at a scale speed 12.5 mph against a maximum road speed of 10.5mph. The tank was easy to control with a steady build-up of movement and did not snatch or jerk. A good result.

[Herr Dr. Professor]

I am shaking my head in amazement. And you had trouble with 0-level physics?! C'mon: you sure can put together a nifty homespun physics experiment. Perhaps this should be moved to the Masterclass section. Let me see if I got the main points correct at all:

1) for slow speed and power, Igarashi 048 in a 5:1
2) second place for slow speed and power Taigen 390 in a 5:1

WARNING: these 2 points of summary may be inaccurate! Do not trust them until enginepeter checks them. I will edit and correct them a.s.a.p after enginepeter corrects me.

[enginepeter]

Thank you Herr Dr Professor for your comments. If I fully understood amps, mNm etc I wouldn't have had to concoct experiments to reduce numbers to the lowest common denominator!

The experiments with the gearbox were with the 4:1 gearbox as I wanted to retain the increased torque that it produce over the 5:1. I have edited the post to make this clear. Thanks for taking the trouble to read it all. Unfortunately the numbers are not clear - they look well set out in the draft!

Yes to summarise the Igarashi 048 gives me the best power at a speed suited to my tank (in a 4:1). The Taigen 390 runs faster and has the ability to give more power subject to the level of amps it consumes which can be over 20 amps.

regards. Peter

[Herr Dr. Professor]

O.K. Peter: now for my really basic question . How does one identify the brand and model (e.g. Taigen 390). This is probably obvious, but for years I have opened every tank I have and/or had, from HengLong through Mato to Taigen/Torro to unidentified, and I have swapped out gearboxes, but I can never figure out a way to tell which motor is which.
Go ahead, my ego is strong: if it's obvious, I will be laughing, too.

[enginepeter]

Hi

The motor I had was fitted to a Taigen 4:1 gearbox and the invoice said it was a 380 (I no longer have the box) so I thought it was a 380! However I believe the body of a 380 is 37mm long and the 390 47mm long and it wasn't until I started to look into the spec of motors that I realised what motor I had!

regards. Peter

[jarndice]

It has been some years since I swapped gearbox motors but they used to be identified by their colour I have no idea if that is still the case.

[enginepeter]

I have only got a small sample of motors but the majority are black and a few silver. I have seen comments about red or blue Taigan motors (390 or 380) but in my limited experience I have not seen these. Peter

[43rdRecceReg]

A fantastic, and laudable bit of empirical research, Peter- in the best tradition of pioneering Brit engineers!. Now, follows- what to do with it- with applying it...
Roy.