Tips/tricks to dial in T-jet brush spring tension?
What is the best way to precisely adjust the brush spring tension in a T-jet chassis?
We all know, not enough tension = poor contact and the car won't run reliably, if at all; too much tension = the brushes bind against the armature and the motor drags, resulting in slow speed... so...
Is just bending the spring up and down, reassembling the chassis every time (tedious), using trial-and-error the only way to dial in the spring tension? It takes forever to do that, and with a few stubborn chassis I seem to be unable to get it just right after 3, 4, 5+ tries.
I am using original Aurora chassis with a few repro parts: new brushes, pickup shoe springs, etc... but mostly all original Aurora parts. Could the issue be the armature? Yes I also clean everything thoroughly, etc (I really know my way around a T-jet chassis).
I set the springs so the brushes sit proud of the base of the chassis pan the same little distance, and they "feel" about the same amount of bounciness when touched with a small screwdriver blade, but then often once the car is back together it ain't right. Getting a bit frustrated!
In part I answered this question in another thread. The brushes should stick up about 1/16th of an inch, compare the car in question to a car that runs well. There are all sorts of possible problems with the armature that can keep the car from running well. The armature can have several defects that impacts on how it makes up with the brushes. The commutator plate can be tilted or warped so that the brushes bounce up and down as the armature turns. Sometimes the commutator is off center and the segments are not always the same length. Each segment should be just short of 120 degrees. There are also possible problems with the stacks, those are sometimes off center as well making the armature magneticly and mechanically out of balance. In an earlier post I mentioned that the poles can have different ohm values. The less of these defects that the armature has the better that it will be. There is one final problem that impacts the commutator to brush contact and that is the fact that in some cars the brush plate is positioned so that the armature is running at an angle. With A/FX and Magnatraction cars there are holes in the side of the chassis and you can see that problem. With the T-Jet chassis the problem is hidden. Some people keep swapping gear plates until they find one that works better. Some people have a fixture that they use to test the alignment.
Was expecting more responses... (Thanks for yours Rich!)
Well, if anyone else finds this thread looking for similar info, here is a great article that pretty much covers it all, including how to test and balance armatures: https://sites.google.com/site/speedi...pancake-part-1
One rule of thumb is to look at a stock "virgin" chassis that hasn't been tweaked, and try to set the brush springs to that level. Then drop in a new pair of Wizzard Tjet brushes and you should be pretty close.
On many of my really hot and tuned Fray style Tjets, I'll actually tweak the brush springs to be just flush with the top of the chassis, on the motor side. Then when you put the brushes in, they are just barely into the hole. This gives a bunch of brush tension, and will really make SOME cars run better....but not all! After the brushes wear in for a little while, the car can be a real rocket. But again, that doesn't work on all cars.
The real key is to be patient and just try MINOR tweaks. You slip and booger up those little spring dudes, and the car is most always done....at least from a racing point of view.
there is a power brush and non-power brush
the non-power brush normally does not need the same tension as the power brush! (same for the shoes)
using a dyno, I get to play with these concepts a lot!
I can setup shoes on the dyno to know if i have good contact
for example, without fail on my cars, i get more speed with slottech brushes than any other!
I can touch the brush spring lightly to see if 1 or both springs need more tension.
Keep in mind copper brush springs have less inherent tension than Tuff One silver, so if you're going by eye alone, you may put too much pressure on a TO. Also, the copper bends much easier, so watch out.
Once the tension is set by eye, put the car up to an old transformer, like from a set from the 60's. Maybe a 9 volt battery will work, though you'd run out of hands. You want to run the car and see if pressing down on the brushes at the bottom of the car, to increase the tension (I use a toothpick) gets you any more speed. If the speed really goes up, you need more. And, maybe just on one brush, so that's very good to know.
If you're really frustrated, have too much pressure (the car is "locking its brakes" after you let off the throttle) without taking it apart you can "massage" the tension springs from the bottom. A crude method though... test on the transformer to make sure you didn't lose too much from one brush.
Rich is right about the armature causing trouble, and sometimes it's easy to see it. Use your fingers or the 9v battery to turn the wheels and motor, and look at the brushes from the bottom. If your tension feels tight enough, but the brushes are still bouncing up and down, then your armature is off. If the comm plate isn't flat, you can sometimes improve it a lot by eying it and pressing around the edges with your fingers. Take the top plate and turn the whole upper gear train and arm by the rear gear, while watching the bottom of the arm, from the side, to see where the arm is off, if you determined this to be an issue from the brush hole view. But this is major armature stuff, an issue on only one out of ten of my cars.
If you just can't seem to get enough tension out of brush spring, you can take a pin size piece of metal, place it under the spring between the metal strip and the chassis, close to the rivet. A bend here will give you a lot more tension, sometimes needed in an overworked chassis. Do that last though, after new brushes.
if your doing it for yourself, this info is great.
if for competitive racing, the concepts are the same but there are some good tools out there that make it easy!!!!! RTHO post expander, flatness block, VRP comm truer, dyno, brush/shoe tension tool and etc....