Camera Arm

The Camera Arm moves the video camera up and down over the surface of the barrel. Its is built as an assembly which supports itself on two legs and securely attaches to the base with one screw.


Camera Arm assembled to base	Detached for shipment

The type of mechanism chosen to move the camera on its vertical axis is a lead screw and nut. (This works like any screw/nut pair: hold the screw stationary except for rotating it between your fingers. Attach what you want to move back and forth to the nut, and when you turn the screw, the nut moves along the screw.) Of the different types of lead screw styles, an ordinary piece of threaded rod was used, primarily because of cost. The 4-foot section of threaded rod has a 1/2-20 thread, giving 1/20 in of travel per turn of the screw. The screw is driven by a 3000 rpm servo motor, driven at a maximum speed of about 2000 rpm. This gives a maximum speed of vertical camera travel of 100 inches per minute, or 1-2/3 inches per second. Rarely is it aesthetically desirable to move the camera that fast, however.

Ideally, one could attach a lead screw directly to a motor shaft and turn it, but there are practical problems in making the simple approach work. As a general rule, motor bearings are not designed to carry large thrust loads, and will expire quickly if subjected to them, but the greatest problem is getting every bearing and rotating part exactly collinear, that is, one imaginary line would run exactly down the center of all the parts all along their lengths.The threaded rod needs support at both ends. Long lead screws not well supported and driven too fast will whip around and bend themselves out of shape. Add to this the fact that the threaded rod is not perfectly straight (that's part of what you pay for with more expensive lead screws), and the need for some "give" in the system becomes apparent. The consequences of not providing either an accurately straight line of action, or some give, is twofold. Binding and bearing wear are the result of the bearings trying to twist or bend the axle which goes through it. Binding is another way of saying "making the shaft difficult or impossible to turn". It only takes a tiny amount of misalignment, maybe a few thousandth's of an inch laterally, or a degree or so angularly, to bind an axle quite firmly. This problem often occurs in situations like attaching a direct driven (as opposed to belt driven) pump to a motor. To facilitate these types of direct connections between shafts, a "spider coupling" can be used. Two metal coupling-halves which do not touch each other are connected by an X-shaped piece of rubber. The rubber transfers torque from one coupling-half to the other. The actual amount of misalignment a spider coupling will tolerate is rather small, however. Using two spider couplings and the coupling shaft (to decrease the angle of misalignment) gave much smoother operation than a single coupling.

In addressing these issues, which can be particularly aggravating when using threaded rod as the screw, careful attention to what needs to be really solid, and what can float around a bit (learned through experimentation) can yield a stable, efficient, cost effective mechanism.

To set up this lead screw required several steps.

cut the threaded rod to length, and clean the burrs off the ends of the thread
mill the nut from Delrin (TM) and drill the hole for the screw to pass through and the mounting holes for attaching it to the sliding bearing
thread the nut with a 1/2-20 tap
put the threaded rod on the lathe and turn down a couple of inches on each end to 3/8" to fit in the bearings (this obviously removes the screw thread along those couple of inches)
cut and drill mounting blocks to hold the bearings on the 80-20
mount the nut on the sliding bearing so that it floats freely
thread the rod into the nut to 1/2 its length
mount the bearings on the 80-20 so that they are free to move
attach the motor, spider couplings, and coupling shaft , and fasten securely
connect the motor to a variable supply with an ammeter and start lining things up

There are two ways to tell how much binding is present. The rough test is to turn the screw with your fingers to see if it has too much resistance. The second is to run the nut back and forth over its length of travel using the motor. The goal is to get everything as snug as possible without drawing too much current. By watching the ammeter on the power supply as adjustments are made, it is possible to see where the gains are. Remembering that the lead screw is not perfectly straight, and will move side to side very slightly as it rotates, it is easy to see why the nut has a tiny bit of float in its mount. This is accomplished by using jam nuts to hold the mounting nuts in place. The nuts were set at their best position and then the jam nuts tightened against their neighbors, thus locking each pair of nuts securely on its mounting screw. The top bearing on the lead screw uses a similar method to allow for a tiny bit of movement as it runs. The bottom bearing on the other hand is supporting the weight of the lead screw and the camera arm, and must be solidly attached. The bottom bearing, really a bronze bushing in a cast iron housing, is experiencing the axial load of the rotating lead screw, and the thrust load of the weight above it. To help distribute the thrust load, a shaft collar is attached to the lead screw, secured by a set screw. Between the collar and the bearing is a Delrin thrust washer. Delrin is to a large degree slippery enough to carry the load without lubrication. But the bushing below it does require oil to cope with the rotation of the shaft.