Plastic Armature Gear Experiances

[Grandcheapskate]

Hi Guys,
I have the need to check out samples of new axles I am having made and while I was at it I decided I would also test out a Dash 4 lam armature I bought quite a while back along with using plastic armature and idler gears. After assembling the chassis using an old Aurora bare tub and top plate, and adjusting the height of the armature I got the car running.

Because the 4 lam armature will Ohm at around 5 it required the (almost) strongest magnets I have. This makes the gear train pretty tight because it takes more force to turn an arm paired with strong magnets. After a number of laps the arm gear lost it's grip on the armature and any subsequent reinstall only lasted a while before getting loose again. I eventually installed a metal arm gear so I could continue testing.

I figure the plastic gear let loose due to two reasons. (1) I needed to reinstall the arm at least twice to get the proper height so the arm would contact the brushes and (2) there is a lot of torque on the gears because of the strong magnets.

I did not try a second arm gear nor did I try to "preen" the plastic gear (if that is even possible).

So here is my question...how has everyone else fared with using the plastic armature gears? Should they only be used with standard 15-16 Ohm armatures and stock strength magnets? Do the lower Ohm armatures and strong magnets over-match the grip strength of the plastic gear?

Thanks...Joe

[HO RacePro]

'Skate,

Another reason the plastic gave up might be the armature shaft heat. The retaining stress in the plastic will relax if the plastic gets warm enough. Once that happens reinstalling the gear won't work -- the hole in the gear will have taken a 'set' at a larger size. There won't be enough retaining stress left.

Plastic just isn't very good at retaining stress, which is why knurled axles were designed into the original T-Jets. A knurled shaft will hold onto plastic parts better than a smooth shaft.

If you have a smooth shaft you're better off with metal gears.

Ed Bianchi

[Grandcheapskate]

Ed,
This particular chassis is generating a lot of heat due to the low Ohm armature combined with the need for strong magnets. As I continue to experiment with this setup I may figure out ways to loosen the drive train which should reduce the heat, although I think using strong magnets means the drive train will never spin freely. Because of the magnets, how much I can squeeze out is the question.

Just as an aside, when I put the weakest set of new magnets I have into the chassis, the arm did not spin at all. I have magnets which range from N27-2 (strongest) to N27-6 (weakest). N27-4 is about equal to stock AW and early Dash.

This is why I am wondering if the plastic gears would have a better chance of holding if the armature/magnet combination is more in line with stock Aurora parameters.

The new TFX chassis use all plastic gears and I have heard nothing but praise about those chassis. And those, as far as I know, use a smooth armature shaft.

Joe

[RichD]

The only cars that I have that use plastic armature pinions are TFX cars and there have been no problems with those. HOCOC has a Modified Madness class where any gears, armature and ceramic magnets can be used. Some of my cars have lightened brass idler and driven gears, but those are expensive. I am expecting to try some plastic gears in the future, but I don't want to have a failure in the middle of a race. Possibly gears made out of different types of plastic are available. I believe that there are gears made out of Delrin and Nylon. Plastic gears can be peened or swaged to get a tight fit, but if heat is a problem that might not be enough to keep an armature pinion gear from slipping. For now, I would be leery of using a plastic armature pinion gear.
I was just working on a car that had a Dash 4 lamination ~5 ohm armature and 1100 gauss ceramic magnets. Ohms does not necessarily tell the whole story with respect to armature performance. The armature did not have much wire on it and the car had lots of top speed, but lacked acceleration, even with a 9 tooth drive pinion. Overheating was not a problem. I switched to a rather expensive 5.8 ohm Typhoon balanced three lamination armature using the same magnets and that had more punch with the same top speed. That combination also seems to run cool enough.

[Grandcheapskate]

When I first put the entire chassis together and put it on the track I got nothing. It took a while but I finally realized the armature did not contact the brushes. I raised the brushes as much as possible and still nothing. So I had to uninstall the armature and reinstall it...this time not inserting the arm shaft as far into the arm gear and using at least one spacer under the gear plate. This got the chassis running until the arm gear let go. I'm guessing a plastic arm gear is only good for one install and once removed it is pretty useless.

However heat is an issue. As Rich points out, there is not a lot of wire on the 4 lam armature. Today I tried stepping down one level of magnet (from a N27-2 to an N27-3) to see the difference. The car seemed to run about the same, maybe with a bit more drive-ability. However, the arm shaft at the bottom of the chassis is extremely hot. I did not touch the metal arm gear at that time although I will do more testing the week.

I could try to install a new plastic gear to see if it holds but if Ed is correct, the excessive heat generated by the combination of low Ohm arm and strong magnets may be too much for a plastic gear to withstand.

Joe

[model murdering]

Conventional wisdom dictates that you smell bacon when you touch it, it's too hot for plastic gears!

Raise your hand if you have any original Aurora T-jet gear plates with tinned armature gears.

[SouthShoreRacing]

Either it is the material being used in the gears or the surface area armature gear contacting the armature shaft. Also, repeatedly installing and removing the gear is going to loosen it up. For inline cars we use plastic pinion and crown gears in most classes including CMPM which has strong motor and traction magnets and armatures under 2 ohms. They have lots of torque and lots of heat (150 - 175 Fahrenheit). Slipped pinions do happen, but usually it's when temps go above 200 degrees. Personally I rough up the surface of the armature shaft before I press the pinion on and I use some blue Loctite as well. I have not had issues with the pinion failing or moving unless the car got too hot.

[RichD]

I am not convinced that using stronger magnets would cause the car to run hotter. In the case of modern inline cars using stronger motor and/or traction magnets would generate more magnetic downforce, but also increase drag leading to higher temperatures. Because there is a lot more distance between the magnets and track rails with T-Jet type cars magnetic downforce is minimal, even with strong magnets.
As a DC motor heats up the resistance of the armature will increase and the power output will drop. If your car is heating up probably you will need to change something. Excess motor brush tension will generate extra heat. Perhaps you would need to change to a smaller drive pinion. Make sure that the car's chassis is not dragging on the track.

[keven]

Possibly gears made out of different types of plastic are available. I believe that there are gears made out of Delrin and Nylon.

I went with the orange delrin (Dash) gear set on a WIZZ orange chassis: B3DC7998-4391-4E75-9B49-86DD0F40F260.jpg
16 ohm arm with an orange (of course) delrin crown gear. No slipping so far! And the lap times are pretty fray like…

[Grandcheapskate]

I toyed a bit with the car today. Got it running pretty good (using a 9 tooth pinion) but it does get hot. One thing I did do was trim the rear collar of the crown gear to allow a bit of side play...it seemed the crown/pinion gear mesh may have been too tight. That may have relieved a little tightness.

The new arm, idler and crown gears I used are Dash so they are Delrin. The crown gear is locked tight on the rear axle so no issues there, even after having removed it and reinstalled to trim the collar.

Let me ask this question...what is the advantage of a four lam 4-5 Ohm armature? Unless I am missing something in the parts or setup I'm using, the performance of the chassis is on a par with higher Ohm armatures. I started to use one of those original Aurora controllers with the dual resistors (I forget the name, is it Controller Plus?) and the chassis does have a nice speed range with this low Ohm controller (I think it is 35 Ohms and slightly adjustable).

Parts list:
Aurora 20v power pack
Aurora Tuff-Ones bare chassis (solid rivet)
Currently using all brass top gears
Dash 4 lam armature
85% copper brushes (.072" height)
N27-3 level magnets
9 tooth pinion
15 tooth crown

Just don't touch the bottom of the chassis after a few laps.

Joe

[RichD]

That Aurora power supply could be part of your problem, the output is probably half wave rectified and is unfiltered, so there as a fairly large AC component. The original Aurora T-Jet armatures have two laminations that are thicker than the ones used with the current Johnny Lightning/Autoworld, Dash and TFX armatures. In theory, at least, an armature with more mass can put out more power, which is why I don't like to grind down the stacks to true them up if they are off center or if the armature needs to be balanced. I prefer to dump arms with off center stacks and use epoxy to balance pancake armatures. The power output is also proportional to the strength of the magnets, the number of winds and the current going through the winds. If you want to increase the current you need to decrease the resistance of the winds, you can do that two ways. You can use less turns or use larger diameter wire. With larger diameter wire there would be less space to have enough turns. With four laminations there might also be less space for enough turns. To complicate things further the old Aurora four lamination armatures had double winds.

[BIG E]

Hello, Joe! Be seeing you in Parsippany on November 13th for the slot car show. Bringing a load of cool stuff to sell, and of course, I'll be checking out what you'll have available.
I usually always find something, right? Take care, see ya then. -- Ernie

[Dyno Dom]

I've used the Dash 14 and 24T gears which work very well for Aurora compatible chassis such as Dash and Wizz Jet previously shown in a nice pic by Keven.The gear mesh is not as smooth on AW/JL as I believe the center distance for arm and pinion shafts differ slightly for that chassis. The installation would be easier if proper anvils were available as these gears have a boss/nub which protrudes on one side of the gears to help support the shafts and prior peening of gears would be suggested.

[model murdering]

Rich D: That Aurora power supply could be part of your problem, the output is probably half wave rectified and is unfiltered, so there as a fairly large AC component.

Good catch! Buzz boxes and wall warts arent recommended for hi performance.

[Grandcheapskate]

Rich D: That Aurora power supply could be part of your problem, the output is probably half wave rectified and is unfiltered, so there as a fairly large AC component.

Good catch! Buzz boxes and wall warts arent recommended for hi performance.

I have a 10 amp 18v regulated power supply, just never got around to wiring it in. Maybe I will now have the time.

Just to let you know, I have one of those Aurora packs per lane so they are not being shared. But I get the point about them being half rectified.

Joe