October 2011 Archives

Positioning pushers are needed with transfer conveyors in order to align and position work-pieces. Especially when the following process utilizes any automation, they are required.

1. Out-feeding of work-pieces by Pick & Place units.
2. Automated work-piece inspection with image processing systems.
3. Processes requiring accurate work-piece positions such as automated soldering stations and resin dispensers.

(1) Focus points of positioning pusher material selections

The mechanism example uses an air cylinder (Pico-Table MPPT16-20-RS) to push and align the position of work-pieces on the conveyor.

1. Lightweight ---- In order to lighten the load on the actuator
2. Not to inflict scratches or dents on work-pieces.
3. Wear resistant properties

(2) Positioning pusher material selections and characteristics

The materials used for the mechanism can be largely classified into the following.

• Metals ---- Ferrous metals, Aluminum, Copper
• Non-metal ---- Plastics, Ceramics

From the selection focus points of (1) above, the plastics are chosen as a candidate, especially with the criteria (1) - 3 engineered plastics are suitable.

When using plastics as parts of mechanisms, the mechanical strengths (tensile and compressive strengths, hardness, toughness) are lower than that of the general ferrous materials used, and the plastic wall thickness must be designed larger.

This section explain the method of detecting workpieces moving on a work transfer conveyor (Photo 1) as a typical application of non-contact type photoelectric sensors.

(1) Detection of moving workpieces

The following three different methods of workpiece detection are available: (1) contact detection, (2) non-contact detection, and (3) close contact detection. Non-contact type sensors are selected here for moving workpiece detection for the following features:

1. No possible damage to the workpiece
2. Capable of detecting workpieces during machine operation

It should be noted, however, that the detection light may behave unpredictably depending on the shapes of the mechanical parts around the sensors and may cause detection errors, leading to a reduced operating ratio of the machine.

- Precautions for use of non-contact type photoelectric sensors

• The light emitters/receivers of the sensors must be kept free of foreign matter, such as debris, dust, water droplets, working fluid droplets, and chips. Install sensors facing in the direction with the least likelihood of adhesion of foreign matter. For example, avoid installing sensors facing upward (so that debris will not fall on them).
• Carefully select the sensor mounting locations to prevent interference between neighboring sensors.
• Shield sensors from incident light emitted from the surrounding structures (external incident light and reflected sensor light).
• Avoid mixing opaque and transparent workpieces. Failure to comply may result in inaccurate detection due to changes in surface light reflectance. (This is particularly important with reflection-type sensors.)
• Select fast response sensors for detection of fast moving workpieces.

Photo 1 shows a work transfer conveyor (second from the right bottom row in "Typical Applications of Standard Components: 1" on the center-page spread of the catalog) with three photoelectric sensors (beam sensors: PEX13B) performing the following functions:

• Detects workpieces when feeding onto the conveyor belt (see Photo 1 above).
• Controls the positioning station in the middle of the conveyor to adjust the orientation of the workpieces.
• Detects the presence or absence of a workpiece at the end of the conveyor and limits the number of workpieces on the conveyor to the specified value.

There are various shaft fastening methods available, including ones used to couple two shafts or to fasten a shaft to a boss for power or torque transmission. This section provides a brief comparative summary of shaft-boss fastening methods for power or torque transmission.

(1) Shaft-boss fastening methods

A mechanical element fastened to a shaft is called a boss. Typical shaft and boss fastening methods are shown below:

-Fastening with keys (See Fig. 1.)

-Fastening with setscrews (See Fig. 2.)

-Coupling with pins (See Fig. 3.)

-Frictional locking (See Sections 92 and 93.)

-Weld-fastening

(2) Comparison of fastening methods

In addition to the tapered sleeved type introduced in Section 141, there are also single and double conical frictional locking devices Mecha-locks available (see p. 993 and p. 995 of the FACE FA Standard Components Catalog).
This section describes the structures and features of the single and double conical Mecha-locks.

(1) Structure and features of single conical Mecha-lock (MLM, MLMP, and MLHS)

Pushing the pressing flange integrated with the tapered inner ring into the tapered hole of the outer ring by tightly screwing the multiple numbers of locking screws into the threaded holes of the outer ring produces friction between the inner ring and the outer ring. (See Photo 1 and Fig. 1.) The radial component of this frictional force locks the motor shaft and the sprocket. The single conical Mecha-lock fits into narrow spaces but can produce a relatively large radial locking force. The single conical Mecha-lock is used for locking various rotating power transmission elements, including sprockets, gears, and pulleys.

(2) Construction and features of double conical Mecha-lock (MLA and MLAP)

Tightly fastening the multiple sets of the two opposing tapered side rings by setscrews produces friction between the tapered rings and the outer/inner rings, and the radial component of this frictional force locks the shaft and the hub (for example, sprocket). (See Photo 2 and Fig. 2.) The double conical design enables applications that require large torque transmission. This type of Mecha-lock is widely used for clamping flywheels, pulleys, etc.