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Real Performance Machinery Screen Printing Article :

Faulty Switches Are Downtime Culprit

Faulty Switches Are Downtime Culprit
By Rick Fuqua, Contributing Editor

Switches on a T-shirt machine allow the press or computer to know when a part is in the right position and/or has moved to a desired place; in short, they confirm the locations of parts. If, for some reason, this message doesn't get across, it can stop a press and cause you to waste hours trouble-shooting parts that work.

For example, when the squeegee/ floodbar carriage is moved through a flood/print cycle, switches at both ends of the stroke are actuated to tell the press's computer, or PLC, the carriage has returned to the right position to start (home) after moving. Failure to reach one of these switches is likely to cause the machine to appear locked up.

In this instance, lack of confirmation that the carriage has reached the other end of its stroke would result in the carriage not being allowed to return. The missing input signal from the switch to the PLC would cause the computer to wait for confirmation before giving the command for the carriage to return. Unless the computer program was equipped with a time-out sequence that shuts down the press after so many seconds of waiting, the press might wait indefinitely for a confirming signal from the switch.

This apparent failure or appearance of being locked up can be caused by any switch anywhere on the press failing or coming out of adjustment. The amount of downtime that results will be determined by your ability to identify the problem switch and its location and how
quickly you can replace the part. If you don't have the part on hand, the time to get it is also a factor. The location of the faulty switch is important because it might be possible to avoid using a print head if the switch is at a station, but not if it's, say, on the indexer drive unit.
Finding a problem switch is much easier if the press has error codes that identify the area waiting for a confirmation. This simple diagnostic feature enables the operator to check to see why the switch has not been actuated.

In some cases, the air on a pneumatic machine simply may be turned off in that area. Or the pressure may be so low that the device has not moved or is moving so slowly that the part has not yet completely reached the switch. The air cylinder could be faulty or the air-line connections pinched. If the press is electrical, the drive motor or circuit could be faulty, not allowing the part to move. Or something may be blocking the part's path, keeping it from reaching the desired location.

It is when a part, such as the squeegee/floodbar, has reached the switch and the press does not continue its sequence that the switch may be thought to be out of adjustment, faulty, or failed. The switch can be tested for adjustment by manually tripping it or, if it's proximity-activated, placing a screwdriver tip in front of the sensor.

Over the years, many switches can and have been used on printing machines, the most common being mechanical microswitches. These switches move in and out or back and forth by coming in contact with a moving part. These switches typically had various styles of actuators chosen to work for the specific application, which tripped a separate switch to turn power on or off. Failure of this style of switch could be found in the actuator itself and/or in the microswitch.

The actuator typically came out of adjustment or caused failure first due to the physical contact it endured every cycle. The microswitch itself, although designed to cycle without as much physical wear, eventually also failed due to the finite life of its mechanical operation.
Because of their limited life expectancy and the adjustments needed for proper operation, the use of mechanical switches has almost completely ceased. Instead, the industry has turned to switches referred to as proximity sensors in many, if not all, of the applications for which mechanical switches previously were used. The cost of proximity sensors has come down over the years, so the advantages they offer over older-style mechanical switches can be incorporated on machines for little additional cost.

Proximity Sensors
The advantage of the proximity switch/sensor is that it is nonmechanical, leaving the device to work without physical limitations and actuator issues. Life expectancy is essentially just that—for life. The way the device works is by "seeing" metal. On any proximity sensor, there is an active surface that radiates a high-frequency, electromagnetic field. When a metal object (target) comes within range of this surface, the field is absorbed or collapsed, which causes the output state of the built-in circuit to change (open or close).

The distance from the active surface (range) at which a proximity switch can read metal is limited and a function of its design. Some distance is desired so the switch won't be activated by other, nonrelated moving parts. The range should be small enough to account for only the intended target, but not so short as to make it necessary to locate the target within a fraction of an inch of the sensor. Most sensors used for applications in our industry have a O-to-4 mm sensing distance. This means the sensor reads only targets 4 mm or closer.
Proximity switches come in a variety of physical styles. The two most popular, which are used extensively in our industry, are tube and block. Tube-style proximity switches are made in various diameters and lengths. Typically, the outside body of the tube or barrel is threaded. These outside threads allow the switch to be easily screwed in or out of an external housing bracket so its location can be set.
In applications where the manufacturer is concerned about the switch being properly adjusted or desires to avoid possible damage to the device, a block-style sensor may be used. These sensors depend on the mounting of the part to be more defined, as their housings normally are not adjustable. Block-style sensors also can be more compact for use in tighter areas.

Examples of both types can be found on some textile presses. For example, block-style proximity switches are used on the print heads of M&R Gauntlets and Challenger series presses, whereas M&M/American's Centurian press uses tube-style sensors for the same application. A factor at play here is available space. The M&R application is more restricted in space at the sensor location than the Centurian print head. Both of these manufacturers use tube-style sensors for up-and-down indexer location and/or print-arm location confirmation. The adjustable housing in these applications is very helpful in setting the proper distance from the sensor to the part without elaborate sensor brackets.

Voltage Supply
A supply voltage must be delivered to the sensor for the switch to function and generate the electromagnetic field. The supply voltage can be AC or DC. For AC-powered proximity switches, rugged thyristors with a bridge and built-in protection circuits are used to switch the output.
For DC-powered proximity sensors, the outputs are switched using high-capacity PNP or NPN transistors with built-in protection circuits. When purchasing a replacement sensor, it is imperative to know whether the circuit is AC or DC and the supply voltage.

A common supply voltage on many T-shirt presses is 24-volt/DC. Proximity switches in these applications are commonly rated for 10 to 30 volts/DC. If the circuit is DC, it is equally important to know and specify whether the sensor is PNP (sourcing) or NPN (sinking). Sinking is negative switching, and sourcing is positive switching.

The output of a sensor that is sinking is hooked up to the common side of the power supply, whereas the output of the sensor that is sourcing is hooked up to the positive side of the power supply.

If you don't understand this, don't worry. Just be aware there are two types of DC sensors, and you must have the right one for your press to work.

Open Or Closed
All switches come as either normally open or normally closed. A proximity switch is said to be normally open when its output is turned on in the presence of a target (metal object). A proximity switch is said to be normally closed when the output is turned off (open) in the presence of a target and on (closed) when it's not. A third type that is not common is known as complementary. Complementary proximity sensors have both a normally open and normally closed output that change states when a target is present.

A common block-style sensor used by M&R for the print head front- and rear-stroke positions is a three-wire NPN (normally open) sensor. These sensors can be ordered with a small LED to tell when the sensor is reading a target (piece of metal). It is possible for the light still to work even it the switch is bad, although this does not happen often.

To test this sensor, first determine if supply voltage is getting to the sensor, then whether the sensor is switching when metal is present in front of the active surface. The three leads coming from the switch are typically brown, black, and blue (brown, +; black, output; blue,-).
Check to see if your switch is color-coded the same. If it is and your machine circuit is 24-volt/DC, first determine if 24 volts DC is going to the sensor: With a volt meter, read between the blue and brown leads. If 24 volts of DC power is getting to the sensor, it should work.
If the correct power is present but the switch does not work, you can check the operation of the sensor by placing the leads of your meter between the brown or positive line and the black output wire. When a piece of metal is placed in front of the active surface of the sensor the meter should show 24 volts. When the metal is taken away, the voltage should stop.

If this test proves negative, the switch is bad. If the test proves positive, the switch is OK, and your problem lies somewhere else—where being a topic for another column.

As with proximity switches, sensor failures are reportedly 90% physical, where something has hit and broken the sensor or its leads. Other failures can be attributed to input voltage spikes, cut wires, and reconnection failures.

It should be noted that some older presses used sensors without short-circuit protection—something that's available today at a very low cost and should be ordered if you're replacing a sensor. This addition equips the sensor to protect itself in the event of a short circuit without destroying the device. An example of an instance that could cause this is when the lead of the sensor gets pulled too far out of the printhead, gets caught on a moving part of the print carriage, and rips the wires out, causing a short circuit in the process.

When replacing a sensor, it is better to use the new lead rather than to splice it into the old one. To use the new lead on M&R machines, you must pull the wire through the head tubing and attach crimp-on-style connectors to make the connection to the indexer leads coming out of the center indexer tube (main shaft). The old lead can be used to tape the new lead to and pull through the tubing. (An electrician's fish tape also is ideal for pulling these leads through head tubing when the old lead is lost inside.)

If splicing is done, joints should be soldered so they won't pull apart when the sensor is moved in the future when readjusting the stroke length. Since these leads constantly are being tucked away inside a head structure then pulled out again, the joint of the splice should be as strong as possible. Taping or using shrink tubing over your solder joint is recommended to protect the wires against shorting out against each other or against metal objects, as well as to provide shielding against possible noise issues that could affect the switch performance.

Real Performance Machinery L.L.C.

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