Re: Elevate sevo. [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Grant Mahalek wrote:
> I was thinking about yours and Garnet's ideas.....what about balancing 
> the gun and barrel.....and then use a sail winch servo with stainless 
> steel fishing leader line in a closed loop(or anchored someway) to 
> control the gun from bouncing up and down??

I don't have a lot of experience with these things :-)
but here's my two cents worth on the subject of elevation systems.

You should always "balance" the gun to take the load off of the lifting 
mechanism.  A couple ounces of lead is all that is needed to balance the 
Tiger-1 (mark III) barrel.

A perfectly balanced gun will want to bounce when the tank is moving or 
shooting, so you always need to "dampen" the gun from oscillating.  I 
use two air springs (surplus car parts).  Since they oppose each other 
(and the gun is already balanced), it doesn't matter how strong they 
are, as long as they a stiff enough to dampen the oscillations.

With a balanced and dampened gun, you still need a mechanism to move it 
up and down. Since the gun is balanced, the mechanism doesn't 
necessarily have to be strong, but it has to be "durable".  The lifting 
mechanism needs to be able to withstand the dynamic shock of the gun 
moving up and down, as well as when you hit something accidentally. 
(Yes, these are tanks and we hit things occasionally.)

It's not the "motor" or "power" that's the problem, it's the "linkage". 
There are basically three types of linkages:  arm, lead screw, and 
winch.  All of these linkages provide positive control in both directions.

If you use a straight linkage, it will transfer the shock directly to 
the servo/motor, so you'll need a strong hub and arm.  My current servo 
uses a geared motor with a 1/2" steel hub, which replaced a 1/4 scale 
servo that had a 1/8" plastic hub.  If you want proportional control, a 
servo with a straight linkage is the obvious choice.

If you use a screw-type linkage, then the lead screw takes the shock 
directly, so make sure it is up to the task (1/4" threaded shaft comes 
to mind).  Although it's possible to make proportional screw-type 
linkages, they are generally used with on-off controls.  These linkages 
can also be made very compact.

A winch-style linkage absorbs much of the shock in the wire (synthetic 
or metal) and the rest by the capstan.  Although they are usually 
controlled by on-off switches, there are winches sold for sailboats that 
provide fully proportional control.  I use one in one of my sailboats 
and it works great, although it cost a lot as well, because it has the 
highest strength-to-weight ratio, which is important for sailboats.

Having built and bought numerous versions of all these approaches over 
tha past 15 years of modeling, my latest choice for the Tiger was the 
servo board from Oakley (for proportional control) and a geared motor 
(for robustness).  My choice for the Navarrone gun was a geared motor 
(for robustness) and an on-off switch control (for simplicity).  Both 
approaches provide accurate fire control and are robustness enough for 
years of abuse.  Geared motors take up more space than servos, but they 
are more robust.

        Frank "Tried Everything More Than Once" Pittelli