Do Leaf Springs Wear out?

In this brief article, we will discuss how the leaf spring suspension works, the wear mechanism in leaf springs.

Do leaf Springs wear out?

Yes, certain types of multi-leaf springs do wear out. The type of leaf spring most likely to wear is the Semi-elliptic type of Multi-leaf spring. 

Basic Working principle of a leaf spring

The Basic working principle of the Leaf Spring Suspension is the Cantilever beam bending mechanism. The leaf spring is constructed as a stack of leafs or slender spring-steel plates that are arranged in a semi-elliptic shape. The stack is tightened at the center using 2 U-Bolts and fastened to the axle at the axle-seat.

In principle, you could consider the leaf spring assembly as 2 Cantilever beams that are subject to ‘Bending’ at either end. At the 2 ends of the leaspring the topmost leaf is rolled into a circular shape. The circular ends are pressfitted with a ‘Pin’ or hinge at each end that are usually serviceable joints that enable the leaspring ends to freely move in the vertical direction. So the bending stiffness of the leaf spring is converted into a vertical directional stiffness of the axle movement.

How does wear Happen in a Leaf Spring?

The Bending Stiffness of the semi-elliptic multi leaf pack will only be available if all of the leaf ends bend together. In traditional truck leaf springs, the leaf springs are cambered to be in contact with each other. In accordance with simple bending theory, as the leaf spring compresses, the upper surface of every leaf is in tension and the lower surface of each leaf is in compression.

At each leaf-to-leaf interface, the lower leaf upper surface is in tension, while the upper leaf lower surface is in compression. This causes a sliding motion between the leafs every time the suspension is in motion. The leaf spring outer surfaces are heat-treated to be hardened and resist wear. But in spite of this, gradual wear does happen and the leaf spring contact surfaces start to dig into each other.

Many SUVs like the Ford F150, have interleaf separator wear  pads that serve to reduce the sliding surface area and maintain contact only at the tip of each leaf’s upper surface. But even this separator pad wears out quickly and the leaf springs eventually start sliding over each other metal-to-metal.

In the case of Multi leaf Parabolic suspension, the surfaces of the leafs are tapered in a parabolic shape and hence are not in sliding contact with each other. The parabolic leafs are almost always of equal overall lengths. Only the leaf ends are in contact. Being of equal length, the relative motion between the leafs is minimal and therefore the wear is not significant.

Apart from Leaf-to-leaf wear, the eye-bushing of leaf springs are subject to wear. Due to the continuous rotation during operation of the suspension, the eye hinges do experience wear.

How can Leaf Spring wear out be prevented?

  • Most traditional semi-elliptic leaf springs are dismantled at regular intervals and serviced by applying graphite solid lubrication between the leafs
  • If the manufacturer provided interleaf wart pads, then these must be replaced regularly

What are the Different Types of Leaf Spring?

The 2 main types of leaf spring are:

  • Semi-Elliptic leaf spring
  • Parabolic leaf spring

The main differences between the Semi-Elliptic and Parabolic leaf spring are:

  • In the semi-elliptic leaf spring, each leaf is of constant thickness or, in other words, the cross-section of each leaf is constant throughout its length
  • In a Parabolic leaf spring, the individual leafs of the pack are each of varying cross-section, except for the center clamping portion, where the leaf spring is flat. The cross-section varies in a parabolic shape from the center to the ends.
  • In a Semi-elliptic leaf spring there is relative sliding motion between the leafs as the leaf spring travels between rebound and bump. For a Parabolic leaf spring, there is almost no contact area between one leaf and the other, except for the center clamping area and the 2 ends, where there is no sliding.
  • The Sliding motion leads to a Damping effect in semi-elliptic, but there is almost no damping in the case of Parabolic suspension.

Conclusion

In this brief article, we will discuss how the leaf spring suspension works, the wear mechanism in leaf springs.

In case of any queries or comments, please feel free to ask.

Frequently Asked Questions:

How do you tell when leaf springs are worn out?

  • On dismantling the leaf spring, if there is pitting, or material removal observed on the leaf surfaces, then it means that the leaf has worn out.
  • If the car or truck sits at a lower height than usual, it means that the leaf springs have worn out and have to be replaced

How long does leaf springs last?

The exact service life depends on the type of vehicle, tonnage and also the kind of operating conditions the truck is subjected to. Buses and Highway vehicles that have leaf springs would get a life of 80-100K miles for a semi-elliptical and 200K miles for a parabolic. This figure would drastically reduce to nearly half if the vehicle is in Off-Road application most of the time.

Do leaf springs weaken over time? What causes leaf springs to go flat?

Leaf springs, or any springs for that matter, undergo “Fatigue” towards the end of its service life. Fatigue means that the leaf spring gradually loses its stiffness. It offers less resistance to wheel motion toward the end of its life as compared to the beginning. Once this happens, the leaf spring sags. This means that the vehicle sits at a lower ride height as compared to when new. 

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