• In design, the designer has to specify the size of the product.
• The size covers different parameter like power transmitting capacity, speed, load or dimensions of the component.
• Usually, the product is manufactured in different sizes to satisfy the need of different customers
• So, the preferred members are used to specify the sizes of the product.
• The system developed by Charles Renard is based on the use of geometric progression to develop a set of numbers.

• There are five basic series denoted as R5, R10, R20, R40 and R80 series. Each series has its own series factor as

  

• The hoisting capacities of cranes are in R10 series while the hydraulic cylinder diameters are in R40 series and hydraulic cylinders capacities are in R5 series.

• The other series called derived series may be obtained by simply multiplying or dividing the basic sizes by 10, 100 etc.

• Preferred members minimize the unnecessary variation in sizes. They assist the designer to avoid selection of sizes in an arbitrary manner. The complete range is covered by minimum number of sizes, which is advantageous to manufacturer and customer

Advantages of preferred series

• The difference in two successive has fixed percentage.
• Provides small steps for small quantities and large steps for large quantities. It is the confirmation with the mode variation found in nature.