11 replies to this topic

[Alchemist]

Someone informed me that a motor has to exert more power to turn 48 pitch gears as opposed to 64/72/80 pitch gears. If this is true, how so?

 

What are your thoughts on this please?  

 

Is this the reason everyone seems to want to run 64/72/80 pitch gears?  

 

Thank you.

 

Ernie

[Paul5097]

Ernie,

 

I think the finer pitch gears are generally easier to set for a smooth mesh. With a smoother, quieter mesh, less energy is lost in the gears.

 

Just my opinion.

[Bill from NH]

You can also make smaller gear ratio changes with the finer pitch gears.

[W. J. Dougherty]

I think the basic premise is accurate. But I think beveled gears have gone a long way to reduce friction even with 48p. 

[Gator Bob]

Hand raised.

 

OK... I'll try.

 

With a finer pitch... less rotational energy is expended to get to the next tooth... Think... Face time and rubbing surface contact area.

[slotcarone]

If the mesh is smooth and not binding, I can't see how this could be possible.

[old & gray]

Higher pitch should provide multiple tooth contact and less sliding contact, this provides better power transfer and less friction.

[Cheater]

Did a bit of Googling on gear pitch and here's an applicable paste.
 
"The determination of the appropriate module or diametral pitch is a compromise between a number of different design requirements. 
A larger pitch is associated with larger and stronger teeth. For a given pitch diameter, however, this also means a smaller number of
teeth with a correspondingly greater likelihood of undercut at very low number of teeth. Larger teeth are generally associated with more sliding than smaller teeth. 

On the other hand, smaller pitches, which are associated with smaller teeth, tend to provide greater load sharing due to the compliance 
of plastic gears. However, a limiting condition would eventually be reached when mechanical interference occurs as a result of too much compliance. Smaller teeth are also more sensitive to tooth errors and may be more highly stressed."
 
Interestingly, the considerations for gear design change significantly depending on the material used for the gears. Metal gears require different design considerations as compared to plastic gears. For example, plastic gears demonstrate less thermal stability, i.e. more dimensional change with temperature variation. I located almost zero information on composite gearing systems as used in many slot cars, i.e. a metal pinion and a plastic spur.
 
Also, it seems the efficiency of gearing does not vary significantly with gear pitch diameter. The differences between gear pitches largely involve tooth strength, quietness, the effects of center to center distance variations, and manufacturing concerns.
 
One point made in several places was that the selection of pitch diameter is often driven by the size of the pinion needed.
 
Anyone researching this topic will find that the subject is extremely complicated, with no easy guidelines or simple answers.

[old & gray]

<snip>
 
Anyone researching this topic will find that the subject is extremely complicated, with no easy guidelines or simple answers.

 

Large companies who manufacture jet aircraft engines have groups of engineers whose entire job is gear design and selection. Small companies have been known to take a successful gear design and adapt the rest of their product to fit around it.

 

If I remember correctly Gunther Faas found a commercially available 64 pitch gear and brought that to slot racing.

[John C Martin]

Anyone researching this topic will find that the subject is extremely complicated, with no easy guidelines or simple answers.

I figured that way, too. Just didn't know how to explain it.

[Phil Hackett]

Generally speaking... the pitch of the gear has very little to do with power loss.
 
The biggest advantage of 64 vs 48 pitch gears, for slot cars, is the wider range of gear ratios available to the racer. That was also the idea of 80 pitch gears Sonic made... they were ahead of their time... in that the smallest "proper" pinion you can get on a 2mm shaft is an 8 tooth in the 64-pitch system... with an 80-pitch gear you had a pinion gear that would give the equivalent of 7 tooth 64-pitch gear... and since the spur gear clearance is fixed by the tire diameter you could run a 49 tooth 80-pitch spur... 7:1 anyone? Anyway, the same idea is for the 48 > 64 pitches as it is for the 64 > 80 pitches: more available ratios in the given space and limitations.
 
Sonic produced several sets of 120-pitch gears for R&D. NO, you can't have them!
 
Gear math can get really difficult very fast.
 
Also know that most slot car motors don't see the friction of the gears. There are other things with a slot car that are much more power-robbing than the gear set, unless the gears are bound up.

[A. J. Hoyt]

The above technical contributions are all valid. All of the normal "rules" for gears were developed for metal gears through geometrical theory combined with manufacturing methods (hobbing, etc.) and are virtually thrown out the window for plastic gears. The reason is the deflection under load.
 
In order to get every quieter and more efficient gears for automotive and truck transmissions, a lot of the analysis is concentrated on the mesh of deflected teeth; as small as those deflections are (micro-inches), they are important to noise, efficiency, and wear.
 
The theory of gears is that they are formed according to an "involute" generated curve shape with the intent being a maximized rolling contact to minimize sliding friction but there are always compromises. If you can create a proper backlash and hold it (to make room for an oil film while ensuring that you can maintain the oil film), you want a little bit of sliding to engage and drag the oil film along so that the metal never actually touches under load to minimize the metal-on-metal wear.
 
The combination of metal pinion into a plastic crown (or spur) gear for slot racing is no accident. This came about from decades of empirical knowledge gathered over "trial and success" on the track.
 
Plastic has natural lubrication properties when run on steel. The pinion HAS to be steel because it encounters teeth from the other gear much more often and would simply wear out if it carried all of that load, taking the crown or spur with it.
 
I have never tried the lighter method of breaking in gears, preferring to break in new gears on the track while breaking in a new motor.
 
And everyone knows that some motors/tracks seem to like a looser mesh than others. This is OK when the gears really only HAVE to live for practice and a race. The limits of variation that we try (between tight and loose) could never be tolerated with gears meant to last a year of continuous service - we do it for performance.
 
Bottom line, lots of compromises made tuning for the "heat of battle". I am really looking forward to the new gear offerings announced recently.
 
Keep it in the slot,
 
AJ