Principles of Dairy Machine Design

Module 7. Springs

Lesson 30
SPRINGS, HELICAL AND LEAF SPRINGS, ENERGY STORED IN SPRINGS

30.1 Introduction

When flexibility (or) deflection in a mechanical system is specifically desired some form of spring can be used. Otherwise, the elastic deformation of an engineering body is usually a disadvantage. Springs are employed to exert forces (or) torques in a mechanism (or) to absorb the energy of suddenly applied loads. Springs frequently operate with high values for the working stress and with loads that are continuously varying.

30.2 Spring

A spring is defined as an elastic body, whose function is to distort when loaded and to recover its original shape when the load is removed.

30.2.1 Objectives of springs

Following are the objectives of a spring when used as a machine member:

1. Cushioning, absorbing, or controlling of energy due to shock and vibration.

• Car springs or railway buffers.

• To control energy, springs-supports and vibration dampers.

2. Control of motion Maintaining contact between two elements (car and its follower)

In a cam and a follower arrangement, widely used in numerous applications, a spring maintains contact between the two elements. It primarily controls the motion.

Creation of the necessary pressure in a friction device (a brake or a clutch)

A person driving a car uses a brake or a clutch for controlling the car motion. A spring system keeps the brake in disengaged position until applied to stop the car. The clutch has also got a spring system (single springs or multiple springs) which engages and disengages the engine with the transmission system.

Restoration of a machine part to its normal position when the applied force is withdrawn (a governor or valve). A typical example is a governor for turbine speed control. A governor system uses a spring controlled valve to regulate flow of fluid through the turbine, thereby controlling the turbine speed.

3. Storing of energy

In clocks or starters:

The clock has spiral type of spring which is wound to coil and then the stored energy helps gradual recoil of the spring when in operation. Nowadays we do not find much use of the winding clocks.

30.3 Commonly used Spring Materials

One of the important considerations in spring design is the choice of the spring material. Some of the common spring materials are given below.

Hard-drawn wire: This is cold drawn, cheapest spring steel. Normally used for low stress and static load. The material is not suitable at subzero temperatures or at temperatures above 1200°C.

30.3.1 Oil-tempered wire

It is a cold drawn, quenched, tempered, and general purpose spring steel. However, it is not suitable for fatigue or sudden loads, at subzero temperatures and at temperatures above 1800°C. When we go for highly stressed conditions then alloy steels are useful.

30.3.2 Chrome vanadium

This alloy spring steel is used for high stress conditions and at high temperature up to 2200°C. It is good for fatigue resistance and long endurance for shock and impact loads.

30.3.3 Chrome silicon

This material can be used for highly stressed springs. It offers excellent service for long life, shock loading and for temperature up to 2500°C.

30.3.4 Music wire

This spring material is most widely used for small springs. It is the toughest and has highest tensile strength and can withstand repeated loading at high stresses. However, it can not be used at subzero temperatures or at temperatures above 1200°C.

Normally when we talk about springs we will find that the music wire is a common choice for springs.

30.3.5 Stainless steel

Widely used alloy spring materials.

30.3.6 Phosphor bronze / spring brass

It has good corrosion resistance and electrical conductivity. That’s the reason it is commonly used for contacts in electrical switches. Spring brass can be used at subzero temperatures.

30.3.7 Spring manufacturing processes

If springs are of very small diameter and the wire diameter is also small then the springs are normally manufactured by a cold drawn process through a mangle. However, for very large springs having also large coil diameter and wire Diameter one has to go for manufacture by hot processes. First one has to heat the wire and then use a proper mangle to wind the coils.