[OCCRA]: Help for teams

[Mike Martus]

My inspector(s) noticed that many teams did not know some basics on design and loads applied by motors to an arm structure. Here is some basic information I use in teaching my build team pre-season.

How to control a load

A load May be defined as any rotational (Torque) or directional (linear) action placed upon an object. Often when these forces exceed the design of the structure they are applied to there are undesired effects to the structure and the functionality is compromised, reduced or even broken. A key point to consider and remember is that due to the design the forces cannot be eliminated without major considerations of the design. However, shifting the load or controlling the load may be possible to retain the design intent and function.

Consider and answer the following:

• Is the design of the mechanism or structure accomplishing the required task?
• Are the materials of the design or structure adequate for the load?
• Would a change in materials solve the load problem?
• Can the load be shifted to another structure?
• Can the mechanism/structure be made more efficient?

Start by answering question #1. If the answer is yes you can proceed to the next steps. If the mechanism is not performing then #1 is the top priority and MUST be redesigned first and the process starts over.

The load is what it is, a load upon a mechanism. The load will not change unless the mechanism and task changes. In most cases that is not going to happen because you have answered yes to #1. Proceed to #2.

Are the materials of the design or structure adequate for the load (#2 and #3)?
Often just changing the size or strength of the structure will accomplish the task of controlling the load. In most cases if the mechanism is well designed this is the easiest answer to the load issue. Increasing the size of say a shaft by 10% will result in a larger than 10% strength increase. The type of material and shape will decide the increase in strength.

The shape of load bearing structures is critical. Example: Tubing is stronger for deflection than a solid rod if the material is the same. The wall thickness of the tubing will determine the amount of flex before the deflection becomes a permanent bend. Materials with right angles are always stronger than flat stock. Right angle materials with rounded interior corners are stronger than sharp interior corners. Placement of the materials to distribute the load (triangles are a great shape) is critical.

Can the load be shifted to another structure (#4)?
Remember the load may NOT be constant and a momentary load given other factors may exceed limitations of the structure. Carefully examine the maximum load in as many scenarios as possible.

Consider using support structures on axles as close to the load point as possible. If supporting two axles driven by chain a simple ¼ alum support in two pieces with a hole and bushing for the axel will suffice. Then both axels share the load. Often by having each separate and joined with screws to ensue proper tension and spacing will ease assembly.

Can the mechanism/structure be made more efficient (#5)?
Adding bushings or bearing on a rotating shaft will decrease friction and increase efficiency lowering the effect of the load.