Choosing a Machine Loading/Unloading System.

A wide variety of loaders--also known as pick-and-place devices, robots or part handlers--are available in many price ranges. They can be linear (lift, transfer and place) or rotational (lift, rotate and place).

The simplest loader is a single cylinder in which parts are pushed into position. Obviously, unloading can also be accomplished in the same manner. A simple rake-off device can unload parts easily. A more complex variation is one that orients the parts as a function of loading. Round parts can be spun into position with stopping rotation using a pin in a hole or notch.

The next simplest loader is a single air cylinder that uses a mechanism to give a lift-transfer-and-place motion with one stroke of the air cylinder. A single shock absorber and a switch at the extreme ends of the travel are needed for complete control. Intermediate positions are also achievable. The main advantage of the vertical model is that the mechanism carrying the gripper jaw and part is above the arm, giving operator access for adjustments or other machine mechanisms. A gripper jaw can be operated off the same valve as the loader, simplifying flow control. No sequencing of cylinders is required, so a single contact that is opened by the forward switch (or a timer) is all that is needed to return the loader and operate the gripper jaw.

The traditional lift-transfer-and-place loaders use a vertical slide with an air cylinder and guide rods. At each end position three devices are necessary for fast, smooth operation-one is a positive stop; two, a switch to indicate the slide's position; and three, a shock absorber needed to slow the speed. A programmable controller or relay logic is necessary to sequence the slide motion in the correct order.

Another way to accomplish these motions is to power the slide with a servo motor, with power transmitted through a belt or ball screw. Encoders are now needed to slow the motor and bring the slide to a smooth, accurate stop. An obvious advantage to this method is that the slide is now programmable so it is easy to change strokes or have multiple stops if needed.

For extremely high-speed automation, pick-and-place units driven by cams are used. Although more costly, precision and reliability are usually better. Linear pick-and-place units tend to be quite bulky and with any cam-driven unit cannot be adjusted for stroke changes. For extremely long horizontal or vertical strokes (up to 20 ft and more) and gantry part-transfer operations, linear actuators are used. Consideration must also be given to choose a unit that is sealed and has a rigid bearing system.

United Index manufactures a unique and versatile cam-guided pick-and-place system. The tool steel cam only serves to guide the motion. Rigidity and accuracy are provided by linear bearings arranged in an XY configuration. Air cylinders, electric or air motors provide power, and sequencing is done via programmable timers. A curved cam path--eliminating the need for shock absorbers and proximity switches--provides rapid, smooth transition from horizontal to vertical motion. Additional features include adjustable down stops, multi-path down positions and walkingbeam type motions. Linear strokes up to 20in. or more are easily achievable. The example shown loads automotive wheel studs into a bearing retainer, with a 2-in, lift, 6-in. transfer and 6-in, down to load four studs at 12 times/mm. and a single shift production of over 4000 assemblies.

This particular operation presented several problems, such as a pick-up opposition four inches higher than the place position. Also, the studs were only 0.007-in, smaller than the insertion hole. Accuracy of the index table and fixtures, along with variations in the studs and housings, made a loader with consistent placement capabilities a necessity. Because this loader has the speed and smoothness required to place the studs, coupled with the dependability required for long-term operations, it was an ideal solution. At this time, over two million assemblies have been completed successfully with this cam-guided loader.