The Parabolic topic got me looking at a manufactures site for information about springs and spring maintenence. I came across this instead. It may be of some interest to us YJ guys.
On another note: A company that I'm familiar with also has teflon pads between each leaf. They are shot peened and tapered ends. 2.5" Yj springs, but with nowhere near the separation that those parabolic springs had. But with this said, I've seen an Australian magazine that performed leaf spring maintenance. I explained disassembling the spring pack, cleaning it all up and putting it all back together again. Riddle me this?? Have any of you ever done that??
Full Taper Springs
What does a full taper spring do?
There are two types of leaf springs are in use on trucks and trailers; multi leaf and full taper.
Multi leaf springs typically use 5 to 12 or more constant thickness leaves which get progressively shorter down through the spring pack.
Full taper springs typically use 1 to 4 leaves of varying thickness all of which are approximately the same length.
With either type, the basic purpose of the leaf spring is the same:
To act as a cushion between the axle and chassis to protect the vehicle and cargo.
And, for springs with eyes, to locate the axle to the chassis and absorb the forces due to acceleration or braking.
Full taper springs offer several distinct advantages over multi leaf springs which account for their rapid growth in popularity.
Improved ride due to decreased spring stiffness which improves the spring's ability to cushion the vehicle and cargo.
Reduction of spring weight by up to 50%.
Types of Full Taper Springs
The full taper spring is currently used on heavy duty trucks and trailers in four major areas.
Front springs on Class 7 and 8 trucks.
Tandem 4 spring suspensions such as Kenwonth, Peterbilt, Volvo White and Reyco.
Single leaf trailer springs.
In addition, full taper springs are widely used on the front of 4 wheel drive pickup trucks and as single leaf helper springs. Full taper springs are also rapidly growing in use on other applications such as front and rear springs on medium duty trucks, single point trailer springs and class 7 and 8 rear single axle springs.
Compare the differences between a multi leaf and a full taper spring.
Multi leaf Spring
Spring "Tapers" by varying the length of leaves of constant thickness
Shot Peen Each Leaf
Apply paint after assembly
Full Taper Spring
Spring "Tapers" by varying the thickness of leaves of constant length
Stress Peen Each Leaf
Apply zinc rich paint to each leaf before assembly
Liners placed between each leaf
These differences are what allows a full taper spring to meet its four (4) major design goals.
Reduce spring stiffness to improve ride quality.
By tapering each leaf in the full taper spring the load or stress is spread out evenly along the entire length of the leaf. In fact, each leaf acts as a separate spring. The multi leaf spring steps the leaf lengths to spread out the load or stress. The leaves depend on each other to distribute the stress although not nearly as evenly as on a full taper spring. Since the full taper spring does a much better job of distributing stress, this allows for a significant reduction in spring stiffness since each pound of steel in the full taper spring is carrying an equal share of the load.
Improve ride by reducing interleaf friction.
The leaves of a multi leaf spring contact each other along their full length. This causes a high amount of rubbing or friction as the spring is deflected. The advantage of this interleaf friction is the dampening it provides; in effect the interleaf friction acts as a shock absorber. For this reason, many heavy multi leaf spring suspensions do not use shock absorbers. The disadvantage of this interleaf friction is ride harshness; the spring may not respond to small changes in load or bumps. The leaves of the full taper spring contact each other only at the center and at the tips. This virtually eliminates interleaf friction and allows the spring to be very responsive to small load changes thereby improving ride quality.
Reduce Spring Weight.
By evenly distributing the stresses in a full taper spring as explained above, along with the special manufacturing processes that are used, it is possible to reduce spring weight by up to 50% over a comparable multi leaf spring. Typical weight savings average 30%.
Increased Spring Life.
The advanced processes used in the manufacture of full taper springs combine to improve vehicle ride, reduce spring weight and at the same time provide increased spring life over multi leaf designs. Stress peening (shot peening while the spring leaf is under load or stress) allows the full taper spring to withstand the higher stresses. Zinc rich painting of each leaf greatly extends spring life by postponing the onset of corrosion which leads to leaf cracking.
Service Recommendations
Troubleshooting
Full taper springs will usually fail in one of two ways; actual breaking of one of the leaves or more commonly by settling or sagging as the spring nears the end of its useful life. If the spring appears to have failed by sagging, make sure that the sagging is caused by the spring and not some other factor. For example, sag on a front spring can be the result of a twisted frame, uneven vehicle loading or a rear suspension requiring repair.
In either case, verify that the spring is not being overloaded; perhaps a heavier spring would be more appropriate.
Other common failures and their causes are:
Failure Causes
Breaks through center hole Loose U-bolts or worn axle seats and U-bolt plates.
Broken eyes Binding spring pins due to inadequate lubrication.
Repair or Replace?
Repair of full taper springs is not recommended. Cracking of one leaf usually means that the others are nearing the end of their useful life. Full taper springs begin to lose arch (sag) as the spring steel fatigues. Rearching will not bring back the spring's fatigue life. Since a typical full taper spring has 4 or less leaves; one broken leaf reduces the number of working leaves by 25-50%. The remaining leaves have been overloaded and overstressed.
Considering all factors, including down time, means that replacing the spring is usually more economical than a temporary repair.
Think of full taper springs as non repairable components.
Installation
Replace in pairs. Failure of one spring usually indicates that both springs are nearing the end of their life.
Handle with care. The high stresses that full taper springs experience means that they are more sensitive to failures caused by corrosion and surface defects.
Protect zinc rich paint from abrasion.
Do not damage leaf edges or surface while installing. Small nicks or marks can cause rapid failure.
Use the plastic or metal liners supplied with the spring to protect the leaf surface from U-bolt plates or other components.
Inspect and replace as required the shock absorbers. Lack of interleaf friction means that full taper springs rely on shock absorbers to dampen spring movement.
Do not reuse U-bolts. Make sure that new U-bolts are installed and torqued to proper specifications. Recheck torque after a couple weeks of service.
Inspect related components for wear; shackles, spring pins, hangers and axle seats should all be in good condition.
Lubricate all pins and bushings as required.
Maintenance
Properly installed a full taper spring will give many miles of trouble-free service. Preventative maintenance covering lubrication, inspection of shock absorbers and checking related components for wear, as well as avoiding overloading will extend spring life.
Product Bulletin Copyright © Dayton Parts, Inc. Used with permission
Leaf Spring Failure Analysis
Springs are a limited life component.
Regardless of how well a spring is maintained or how favorable the operating conditions are, all springs will eventually fail from fatigue caused by the repeated flexing of the spring. Once the spring life limit is reached a fatigue failure will or has occurred.
Factors influencing fatigue life :
Overloading
The higher the loads or deflections seen by a spring, the lower its fatigue life.
Shock Absorbers
A properly functioning shock absorber will tend to reduce the spring deflection as the vehicle hits a bump. Lower spring deflections mean lower operating stresses on the spring which in turn gives longer fatigue life. This is especially true for full taper springs which do not have the high interleaf friction to help dampen spring deflections. Worn or missing shock absorbers must be replaced to maximize spring life.
Brake Adjustments
Improperly adjusted brakes can also reduce spring life. Under braking, springs are expected to absorb some of the braking forces. If the brakes on an axle are unevenly adjusted one spring will have to absorb more than its share of braking force which can reduce its fatigue life.
Protective Coatings
Corrosion is one of the major factors in reducing spring life. Proper paints and care during handling and installation can help to slow the spread of spring corrosion. On full taper springs the only acceptable coating is the individual painting of each leaf with zinc-rich paint. This paint may be recognized by its characteristic gray color.
Surface Condition
The condition of the spring surface also has an effect on fatigue life. Generally, a fatigue crack will start at some sort of surface defect on the spring leaf. Therefore, care needs to be used when manufacturing and installing springs to reduce these defects to a minimum.
Shot Peening
Extensive testing indicates that shot peening can increase the life of springs by a factor of three or more. It is not enough, however, to simply shot peen the first one or two leaves in an assembly-all leaves must be shot peened. All major vehicle manufacturers specify that their OEM springs have each leaf shot peened.
Decarburization and Steel Quality
Improper manufacturing methods can also reduce fatigue life. For example, poorly controlled heat-treat furnaces can excessively decarburize the leaf surface. Decarburization is the loss of carbon from the steel surface which will result in a soft leaf surface once heat-treating is complete. This soft layer will not be able to handle the spring stresses and will lead to early failure. Poor steel quality can also influence spring life. If the steel has excessive impurities in it, the fatigue life will be reduced.
Maintenance
Finally, improper maintenance will affect spring life.
Spring eyes and other suspension components should be regularly greased to prevent binding.
U-bolts should never be reused.
Axle seats, top plates and other components should be periodically inspected and replaced as required.
Spring failures may be categorized into three types :
Early Life Failures
These type of failures occur generally due to a spring defect, installation problem or overload. This may be due to the material used, the manufacturing processes or improper installation techniques. This type of failure may also be caused by a short-term overload condition.
Midlife Failures
Once the spring has passed the time in service which would expose early life failures, a very low failure rate should be observed, assuming the spring is subjected to normal service.
Late Life Failures
At this point, the frequency of spring failures will tend to increase rapidly as the useful life of the spring has been reached. By this time the spring steel has been fatigued and corroded to a point where its useful life is over.
Failures occurring in early and midlife of the spring are usually most economically handled by repairing the broken leaf rather than replacing the spring. Failures in older springs occur at a point when all leaves have reached their fatigue life the spring should now be replaced. The difficulty, of course, is determining what type of failure the spring has experienced. Basically, the condition of the spring, as well as its service history, will indicate if the spring should be repaired or replaced.
When To Repair
If the spring has not been repaired or repaired only once. Stamping a 1 in the clip for the first repair and a 2 for a second repair will help identify the number of previous repairs.
If the spring mileage is less than half of normal life.
If the repair cost is less than 1/2 the cost of a new spring.
If no more than two or three leaves are broken.
If the failure is not of a fatigue type. For example, a leaf broken through the center hole is caused by improper spring clamping brought on by loose U-bolts or worn axle seats, not fatigue. This spring should be repaired, if possible, and the cause of failure corrected.
Even when it appears to make sense to repair, the following should be kept in mind :
Repair leaves are usually not shot peened and must often be heavily hand-fit to match the old spring. Therefore, the repair leaf will not be as durable as a leaf in a new spring would be.
Since the remaining leaves have lost some of their strength, the replaced leaves will be carrying more of the load than they were originally designed for.
When the leaves first broke the remaining leaves in the spring had to carry more load and were probably overstressed.
Replacing the broken leaves does nothing to restore the fatigue life of the reused leaves. These leaves will continue to fail since their fatigue life is essentially over.
When To Replace
The spring has already been repaired once or, at most, twice.
The spring service mileage has exceeded 1/2 its normal life.
The repair cost exceeds 1/2 the cost of a new spring.
More than two or three leaves are broken.
If small fatigue cracks can be seen running across the leaf width near the U-bolts on the unbroken leaves.
If the leaf tips have separated away from the leaf above.
Never attempt to repair a full taper spring.
Dayton Parts Bulletin G1000692 Copyright ©Dayton Parts 1992 Used with permission
