SEED Guides Sub-System Specification - Planar Mechanisms < >

The purpose of this guide is to enable the user to determine the attributes needed of a planar mechanism to meet a specific combination of output requirements and then to select the appropriate type. It has been compiled as part of a series which covers typical elements of a system.

In this guide only planar mechanisms driven by "constant" velocity rotating Input are considered. Other aspects of mechanism design will be dealt with in subsequent guides.

Figure 1 shows a generalised procedure for selecting elements in the system. It shows that it is necessary to determine information about the drive system before the selection can take place.

Figure 1 General Selection Procedure

SELECTION PROCEDURE
Before embarking on the selection procedure it is necessary to ensure that the need for a mechanism, as distinct from other forms of drive, has been carefully considered. The Mechanical Power Transmission guides may be relevant to some applications.

The successful selection of a suitable mechanism is the result of matching the required output motion to the characteristics of the various types of mechanisms. In general the unit will need to be designed and manufactured specifically for the application: in some cases standard components can be bought from specialist manufacturers.

The most important information is usually related to its purpose, constituent elements, life, performance, environment and the economic situation for which it is required. This information is needed in order to understand the total system so that the mechanism selected is consistent with the rest of that system. The temptation for the engineer to consider only purpose, performance and perhaps environment should be strongly resisted. Although this stage is sometimes difficult and time consuming it must be completed thoroughly if an appropriate mechanism is to be identified.

Factors which influence the choice of mechanism must now be identified. A listing of most of the important and common factors appears below. Not all of these factors are important on every occasion so careful study of the system is required to ensure that those considered are actually relevant. Refer to the Product Design Specification for the system.

Each factor should then be defined in terms which are as objective as possible. Thus, where appropriate, numerical information should be given, terms must be explained and vagueness avoided. Then the boundaries of satisfaction must be defined for each of the chosen factors. This helps the designer to decide which mechanisms meet the requirements in each respect. The boundaries must be numerical whenever possible. When subjective judgements are necessary a means of comparison must be established.

The output requirements and space constraints should now be quantified. Construct a diagrammatic timing chart (Figure 1) and a layout drawing showing the input and the output path. These should not be finalized until the mechanism type has been frozen.

Particular attention should be given to:-

1. operating speed, limiting acceleration and the external load characteristic
   
   
2. requirements for positional accuracy, intermediate precision points and absolute dwells.

Cam-driven indexing units are commercially obtainable; but in most cases the user will need to prepare the detail design, possibly with some commercial components. Detail design is assisted by design data and is likely to require consultation of specialist texts.

There is a good case for maintaining a 'rolling' catalogue library or data on reference cards (or microfilm/computer) since this stage can be very time consuming if a unique set of data is collected separately on each occasion. Data on, say, size, performance, cost can be noted in numerical form, giving a range where appropriate. In the case of less objective data a rating may be shown based on advice or opinion gathered.

Optimising the choice involves finding the best compromise (in the judgement of the designer) between the priorities of the system and the attributes of the various types of mechanism. Some requirements yield a "go/no-go" situation which will eliminate types which are unable to provide absolute dwells, inadequately balanced, too expensive, etc.

Other requirements involving more subjective data should be compared on the basis of their ability to meet the criteria as laid down in the Product Design Specification. This maybe an iterative process which converges on the best compromise.

The evaluation technique used here will be similar to that used elsewhere in the design activity.

Having defined and quantified the requirements and chosen the mechanism type the information needed for detail design is found from specialist design guides or books (see Sources of Information)