Servo - December 2009

screws don’t require tapped holes since they cut their own thread as they’re fastened. They’re good for wood and sheet metal, but they’re a bad option to fasten thicker sheets and plates in the robot structure. They’re made out of low strength steel, and they’re easily knocked off due to the lack of nuts or properly threaded holes.

• Sandwich mounts — These are basically two screws held together by a piece of rubber or neoprene. Besides the male-male version, there are also threaded ones such as the female-female and male-female. They are excellent dampers to mount the electronics into the robot, leaving it mechanically and electrically isolated from the structure.

To hold the screws, nuts and washers are used. Washers are important to evenly distribute the force of the screws onto the part. Nuts have the inconvenience of needing two wrenches to be tightened: one open-ended to hold the nut and another open-ended (or Allen) for the screw head. To avoid that, robots make use of threaded holes.

A hole is drilled in the piece with a diameter a little smaller than the one of the screw (there are specific tables for that). A tap is used to generate threads. Threaded holes make the robot assembly much easier because you don’t have to deal with nuts, which can be hard to reach and secure during a quick pitstop, or that might fall inside the robot.

The thickness in the piece being tapped should be at least equal to the thickness of the nut that would

be used with the screw. This helps to avoid stripped threads. In addition, avoid tapping low strength aluminum (such as 6063-T5) and Lexan, as their threads will have relatively low resistance. Also avoid tapping deep holes in titanium by hand. Besides being tough to tap, there’s a good chance that the tap will break inside the piece.

A rule of thumb for a good screw diameter is to make it a little smaller than the sum of the thicknesses of the parts being joined. For instance, to fasten a 5 mm thick plate to a 4 mm one (totaling 5 + 4 = 9 mm), an M8 screw (with 8 mm diameter) is a reasonable choice.

What about the number of screws? In robot combat, the word “overkill” doesn’t exist; it is just a matter of your opponent super-sizing his/her weapon for your armor to suddenly become undersized.

Therefore, the most critical parts should have the largest possible number of screws, but use common sense. If you drill too many holes to use more screws, your plates will look like Swiss cheese and they will be weakened. A rule of thumb is to leave the distance of at least one washer diameter between the washers of two consecutive screws. In other words, the distance between the centers of the holes should be at least twice the diameter of the washer.

Screws shear much more easily than they break due to traction forces. Therefore, pay attention to the forces that would most likely act on each part of your robot. For instance, in Figure 2 two parts are joined using a screw to transmit a vertical force. The configuration with the horizontally mounted screw is a bad idea, since it will be loaded in shear. Change the design so that the screw will be under traction, as on

the right. In this way, the screw will be able to take up to twice the load.

Another important thing is the tightening torque of the screw. Impact forces are transmitted entirely to a screw that is loosely tightened and it will end up breaking. A well tightened screw, on the other hand, distributes the received impact loads evenly through the surrounding material, receiving just a smaller portion of the impact force. This results in a structure with greater stiffness and strength.

Always check for loose screws during a pitstop. Usually, open-ended and Allen wrenches have an appropriate length (lever arm) for a single person to be able to manually generate appropriate tightening torques without leaving it loose or breaking the screw. A torquemeter can be used to deliver a higher precision when tightening bolts.

Now, how do we guarantee that a screw won’t get loose during a match? The tightening torque by itself is not enough to hold the screws in robot combat since vibration and impacts are very high. A well tightened screw from the top cover of our spinner Titan ended up getting unscrewed after four full turns, until it was knocked off by our own weapon bar. To avoid this, there are five methods:

FIGURE 2

• Spring lock or Belleville washers: These guarantee that the screw remains tightened, working as a spring to load them in the axial direction. Most of the time you can tighten the screw until these washers become flat.

• Locknuts have a nylon insert that holds well onto the threads of the screws, resisting vibration and holding in place anywhere along the threads of the mating part. The locking element also limits fluid leakage and it won’t damage or distort threads.