Engine Crankshaft

Automation Design Tips: Direction Change Mechanisms

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.

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:

a) Select appropriate fit dimensions for pin diameter/hole and shaft diameter/hole for smooth movement
b) Adopt abrasion-resistance measures

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”.

Shaft and hole fits

When rotation axes are located on the same plane and placed at 90 degrees or more to each other, rotation of the driving shaft can be transmitted to the driven shaft by connecting the two axes using a coil spring. (See below figure for details.)

Since the direction-changing unit connected with this coil spring can be designed with some flexibility as long as the driven shaft axis is 90 degrees or larger, it will be utilized for mechanisms requiring assembly adjustment or sorting by the products’ grade and more.

[Fig.2] illustrates examples of a driven shaft connection for a mechanism compatible with various models.

Because a coil spring is used for connecting the component, the following disadvantages must be kept in mind when designing this mechanism.

Disadvantages (limitation of rotary diversion performance)

  1. When the angle (θ in Fig.1) of the two axes become small (less than 90 degrees), the rotary transmission becomes unstable.
  2. Adopting this mechanism to a system involving high-speed rotation or varying rotation speed will result in less precision.
  3. The helical direction of a coil spring must be same as the direction of shaft rotation.
  4. The coil spring may fracture owing to fatigue.

Application examples

  1. Rotation adjustment mechanism compatible with various models
  2. Transmission mechanism having a small rotary torque
  3. Simple adjustment mechanism in a rotary system

It is easy to design simple tasks but adding specifications such as reducing weight and changing direction can be difficult. We hope these tips give you new insight into designing automation or give you inspiration for the next big design. Be sure to read our other design-focused posts such as reducing weight, rotary-linear motion, and large automation!

Cover image from CarThrottle

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