Ramming Speed 2: Drive Train

Motor Spindles

I will be using 1/2″ Team Delta spindles custom made to fit the Dewalt 18v Gearbox. The spindles are keyed, to allow for a secure connection to the sprockets. It’s made of ETD150, a modified chromoly steel that is not super hard but in fact, quite tough – yielding a 150 KPSI part that can be cut with a hacksaw. Hooked up to a special jig it failed at 2.2x the max output torque of the Dewalt transmission giving you a 120% safety margin.

Description: 18v Dewalt Drive Shaft
Vendor: Team Delta
Part Number: RCM512

Motor Sprockets

I don’t believe I need any gearing, so the sprockets on the motors and the wheels will have the same number of teeth. To keep them small and light, I will be using 11 tooth finish bore sprockets that will have a 1/2″ bore to fit the spindles. The sprockets will be keyed to fit the key in the spindles, and will also have set screws to prevent side to side movement.

Chain

I will be using #35 chain (3/8″ pitch). I’ve used this before, it is strong, and it has never caused a problem. The chains will not have removable links, since these can accidentally come off. I will instead properly attach the links permanently.

Wheel Sprockets

The sprockets for the wheels will be the same as those for the motors. They will also be keyed and use set screws.

Axles

In RS1 I used 3/4″ axles to prevent them from being bent. In RS2, since I will be using softer wheels with low clearance (see Wheels below), the chances of the axles being bent are null (the rubber will compress before any force is transmitted to the axle. The axles will be 1/2″ in diameter.

Wheels

Wheels are one of the biggest decisions in a bot, and their are many decisions to be made in choosing the correct ones. Here are some of the issues to be decided:

Diameter

The diameter of the wheel becomes part of the gearing of the drive train. The larger the wheel, the faster the bot will go given the same RPMs from the motor.
Larger wheels will generally weigh more, given more materials are required.
Larger wheels will generally raise the center of gravity of the robot, especially if the bot is invertible (height of the center of gravity generally then linked to the center of wheel).
Larger wheels provide a larger target, especially for horizontal spinners (small wheels may be to low for spinner to hit). Small wheels may also be able to avoid the spike strips (assuming their aren’t bent spikes).
Width

Their is an ongoing debate over what effect width has on traction. It is my belief that width, when combined with a soft compound, will increase traction through adhesive friction (see a discussion of traction in my Tips/FAQs area).
Composition

The materials the wheel is constructed of are obviously very important in a combat robot. Pneumatic wheels will tend to go flat due to damage, and are generally foam filled to avoid this. ‘Mold On’ wheels generally have a solid core, with rubber or polyurethane molded onto it. Colson wheels are this type, and have proved to be very successful. Rubber tends to be the better choice when it comes to the wheel surface, the softer the better. The softer compound can ‘grip’ better due to adhesive friction.

Considering these factors, and my design criteria, I’ve settled on a non-traditional item for use as a wheel. I found rubber rollers, used as spindles in spindle sanders. They are 3″ in diameter, 4 1/2″ long, and have a 1/2″ hole for a spindle. The rubber is very tough, and has a low durometer (guessing around 50 Shore A). The typical Colson wheel has a durometer of 65 Shore A, so these should offer more traction than RS1. I will cut them to a 2″ thickness.