Types of Suspension Springs
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Leaf springs are one of the oldest forms of damping materials ever made. They are very simple in design with a non-complicated construction. They are simple flat plates of optimum thickness held together by means of two U-bolts and a center clip. They can be a mono-leaf or a multi-leaf type. In mono-leaf type there is a single leaf whereas in the multi-leaf type there is a layer of springs, one over the other with gradual change in their lengths from the bottom to the top. Rebound clips are provided to keep the leaves in alignment and prevent lateral shifting of the plates during the operation. Multi-leaf springs are widely used for automobile and rail road suspension. It consists of a series of flat plates, usually of semi- elliptical shape. Semi-elliptic leaf springs are almost universally used for suspension in light and heavy commercial vehicles. For cars also, these are widely used for rear suspension.
The springs may be either cambered initially or flat. Highly cambered springs provide a soft suspension, but they also increase the tendency to yaw (movement about vertical axis). Flat springs reduce the tendency of the vehicle to dip, i.e. pitching when, braking or accelerating suddenly. Use of longer springs gives a soft suspension. Generally, rear springs are kept longer than the front springs. This causes them to vibrate at different frequencies, which prevents excessive bounce. The springs are provided with metallic or fabric covers to exclude dirt. The covers also serve to contain the lubricant used in between the spring leaves. The leaf springs require lubrication at periodic intervals, after every 1000 km. with SAE 140 oil. No lubrication is required in case of rubber bushings.
Tapered Leaf Springs:
The tapered leaf springs are the modified forms of the conventional leaf springs. They have the following advantages over the conventional leaf springs due to which these are becoming increasingly popular compared to constant-section conventional leaf springs.
- Light Weight- Nearly 60% of the corresponding conventional spring.
- There is no inter-leaf friction in case of single taper leaf spring. Even in case of heavy vehicles where more leaves may be required, the number of such leaves is still less than in case of conventional springs. Further these rub against each other only at the ends. Due to these reasons even in multi-taper leaf springs relatively less inter-leaf friction will be present.
- Absence of squeaking.
- The stresses are lower and more uniform compared to the conventional springs, thus giving longer life.
- They occupy less space.
- In case of single taper leaf spring, there is no collection of moisture between the leaves and hence no fatigue due to corrosion damage at the asperities of contact surfaces.
The coil springs are the most commonly used springs in light duty vehicles. They are mainly used with the independent suspension, though they have also been used in the conventional rigid axle suspension as they can be well accommodated in restricted spaces. The energy per unit volume is almost double in the case of coil springs than the leaf springs. The coil spring is a length of round spring steel rod that is wound into a coil. Unlike leaf springs, conventional coil springs do not develop inter-leaf friction. Therefore, they provide a smoother ride.
Torsion bar is simply rod acting in torsion that takes shear stresses only. These are made of heat treated alloy spring steel. Torsion bar is fixed at one end to the frame of the vehicle. At one end, the torsion bar is fixed firmly in place to the chassis or frame of a vehicle where as other end is attached to the axle, suspension arm. The force produce due to the vehicle create the torque on the bar.
Torsion bar spring is lighter as compared to leaf spring and it also occupies less space. Sometimes torsion tubes are used instead of bars, the former being stiffer than the latter ones.
There are also two main disadvantages of torsion bar suspensions. The first is that it does not take the braking or driving thrust so that additional linkages have to be provided for that purpose. The second disadvantage is the absence of friction force, and hence of damping which is a necessity to control the vibrations produced due to road shocks.
Rubber is widely used as a damping material due to its high elastic properties. They provide good results when used in vehicles due to their following advantages:
# It can store greater energy per unit weight than the steel. For this reason rubber springing system can be made more compact.
# The rubber has excellent vibration damping properties.
# The absence of squeaking which is always present in steel springs
# The number of bearings is reduced considerably for the rubber suspension system. This means longer life.
# Rubber is more reliable. A rubber suspension cannot suddenly fail like the metal springs.
Hydraulic Spring (Shock Absorber):
A springing device must be a compromise between flexibility and stiffness. More rigid spring could not able to absorb shocks where as more flexible spring remain continue in vibration mode. So we must have sufficient damping of the spring to prevent flexing. Damping is created due to the friction between the leaves of a spring. In case of coil springs, the provision of damping is completely done by the shock absorbers.
The shock absorbers thus control the excessive spring vibrations. Shock absorbers are a mechanical device that absorbs or dampens shock and dissipate kinetic energy. They are basically of two types, Friction Type and Hydraulic Type. The friction type has become obsolete due to non-predictable damping characteristics.
In hydraulic shock absorber fluid in a cylinder is forced by piston and thus resistance to the piston movement is developed that provides damping effect.