October 2016 Archives

The direction-changing mechanism is a mechanical component that alleviates restrictions such as arrangement or size on the mechanism design when transmitting forces from the driving shaft to the driven shaft. In other words, this mechanism changes the direction of forces or motion from the driving shaft without transmitting them as they are. In the next two volumes, we will study the typical examples of a direction-changing mechanism.

(1)Direction-changing mechanism using bell-crank

[Fig.1] is an example of the direction of motion changing between two links on the same plane of a 90-degree bell-crank.

The bell-crank shown here is a component consisting of two links connected at a certain angle on the same plane. In [Fig.1], the vertical reciprocating motion of the driving shaft (2) is transformed into the horizontal reciprocating motion of the driven shaft (3).

[Fig.2] illustrates the direction-changing mechanism (crank) adopted for automobile engine parts.

= Cautions on designing direction-changing mechanism using links and a bell-crank =

In the case of [Fig.1], forces from the driving shaft will be transmitted through the pin (4) to the bell-crank. The forces rotate around the shaft (5) and travel to the driven shaft (3) through the pin (6). When designing this structure, the following two points must be kept in mind:

The table here shows a dimensional relationship among pin diameter, shaft diameter, and the corresponding hole for the three types of motion statuses. All dimensions are based on the "clearance fit".

For how to select fit dimensions, details will be introduced in another volume.

In the next volume, we will learn about the direction-changing mechanism using springs.

When designing an intermittent motion mechanism using a partially toothed gear, it is necessary to install a locking mechanism (such as nails or hooks) so that the driven shaft side will remain stopped after rotating intermittently by the forces transmitted from the driving shaft. In this volume, we will look into an easier and more convenient method using a friction wheel, eliminating the brake system from the mechanism.

(1) Intermittent motion mechanism using friction force

As shown in [Fig.1], a notch on the driving shaft side (1) is designed to generate intermittent motion. On the driven shaft side (2), a driven pulley with a rubber ring installed is secured around the outer periphery of the circular disk for increased friction resistance. When they come into contact under certain contact pressure, it will generate intermittent motion in accordance with the shape of the notch designed on the driving shaft. Although this mechanism does not require a brake system on the driven shaft side, it is not designed for movement requiring large driving forces or precise intermittent control.

Cautions on adopting intermittent rotary mechanism using friction wheel

Since the rotation force is transmitted by the friction resistance occurred when the driving and driven shafts contact each other on their periphery in this case, care is needed with the following:

(2) Intermittent motion mechanism using friction force

Only two components are required for this mechanism. Both of them are available in the standard components lineup of MISUMI FA mechanical parts. For the driving shaft (1), pulleys for round belts (e.g. MBRM) can be used and a periphery alteration made. For the driven shaft (2), conveyance pulleys (e.g. O-RING type: PFCB or UMHS) can be used.

From the next volume, we will look at typical direction-changing mechanisms that will divert the forces on the driving shaft side.

This volume introduces an automation clever mechanisms that converts motion transmitted from the driving shaft rotating at a constant speed into intermittent movement involving periodical start and stop motion.

(1)Description of intermittent motion mechanism

A partially toothed gear, which has a missing tooth, is used for the mechanism that converts uniform rotary motion into intermittent movement. (See [Fig.1].) This intermittent rotary motion mechanism using a partially toothed gear can transmit power and control the timing of rotary motion; however, there are some points to consider since this mechanism uses gear wheels.

=Cautions on adopting intermittent rotary mechanism using partially toothed gear=

Since the friction resistance between the two gears changes drastically when the intermittent motion starts and stops, be careful with the following:

(2)Typical example of this mechanism

[Fig.2] illustrates an application that uses the partially toothed shape of a bevel gear.

While the intermittent rotary conversion mechanism shown in [Fig.1] is a power transmission between the parallel axes, the transmission mechanism shown in [Fig.2] changes its axis direction to 90 degrees. To design this mechanism, remove a part of the teeth on the driving shaft of a bevel gear and process this part to have a conical surface. For the other gear as well, process the area that contacts the missing tooth part to have the same finish. Rotary motion occurs in the toothed area, but the gear wheel on the driven shaft side will stop when the missing tooth part is in contact with the other gear, thus transmitting intermittent motion.

In the next volume, we will look into the intermittent rotary motion mechanism using friction transmission, which is easier and more convenient than using toothed gears.

In this volume, we will look at an automation clever mechanisms that converts rotary motion into oscillating motion in the horizontal direction.

[Fig.1] is a typical diagram of this mechanism. The basic mechanism design is explained by using this mechanism.

When a crank (1) is connected rotating around the 0 axis (drive shaft) to another crank (3) that will swing around the other rotation shaft called the 0' axis using a connecting link (2) in the link mechanism, the crank (3) starts swinging from side to side as the crank rotates around the 0 axis by one turn. At this time, the swinging range of the crank (3) (from point A to B) is determined by the position where the crank (1) and the connecting link (2) are arranged in a straight line.

In [Fig.1], the two components are arranged in a straight line at point a. The swinging crank (3) inverses its motion after reaching point A located on the right edge. Again, they are arranged in a straight line at the point b, which determines the swinging range point B.

[Fig.2] is the conversion mechanism with an adjusting screw, allowing quick adjustment of the swing angle. This mechanism is useful for performing position control during operation or designing a simple structure compatible with various models.

Tips on selecting appropriate standard components

The parts (1) through (3) shown in [Fig.1] can be assembled by the MISUMI FA mechanical parts. For parts listed in [Fig.2], the MISUMI FA mechanical parts for (3) connecting link can be used if an alteration to the slotted hole is made.

Next time, we will be studying the mechanisms of converting uniform rotary motion into intermittent rotary motion (at irregular intervals).