Figure 1. Boots of standard stitched construction meet demands of most applications cost effectively, especially in small to medium quantities.
Although simple conical-shaped boots are sometimes used to protect short-stroke cylinder rods, a round bellows made by one of several construction methods is generally the most suitable design. Bellows originally were made by sewing discs of leather or rubberized fabric together. The outer circumference of one disc was sewn to the outer circumference of a second disc. The inner circumference of this second disc was then sewn to the inner circumference of a third disc. The outer circumference of the third disc, in turn, was sewn to the outer circumference of a fourth disc. This alternating pattern continued until the required length was attained. This process is still widely used today, but other methods of attachment in addition to sewing are often used. Today&#;s suppliers can furnish a wide variety of configurations and materials to protect cylinder rods and seals from most types of contaminants.
The selection process, of course, requires weighing the interrelated design and operating parameters to pick the best solution for the application at hand. The following variables should be considered:
&#; space available for the boot in both retracted and extended positions
&#; cross-sectional area available
&#; possible interference points along extended length
&#; stroke length
&#; rate of travel and cycle frequency
&#; ambient and operating temperatures
&#; type, source, and amount of contaminants
&#; possible need for venting because of high travel speeds, and
&#; need to accommodate a non-linear travel path because of unusual mounting situations or operation in conjunction with other mechanisms.
Despite all these variables, standard rod boots are available to handle most common protective requirements, and custom boots can be developed for more complex situations, usually with little or no tooling charge.
Different boot constructions offer different extended-to-retracted (E/R) length ratios. In general, as boot OD increases while holding boot ID constant, the extended-to-retracted ratio increases. If radial clearance is limited around the cylinder rod, the boot may have too long a retracted length due to OD limitations. To overcome this, the boot can be mounted over the cap end of the cylinder so it encompasses the entire cylinder. This essentially doubles the retracted length, which effectively cuts the required E/R ratio in half to allow a smaller OD for a given ID. For rodless cylinders, the entire cylinder usually is enclosed by the boot.
Standard stitched-construction boots, Figure 1, protect internal components from damage by common contaminants and nominally severe environments. Assembled from rings of elastomer-coated fabric sewn along alternating inner and outer circumferences, these boots will exclude most dust, dirt, chips and coolants. For added sealing or abrasion protection, seams can be coated after stitching. This construction offers the widest choice of materials for all types of service requirements and allows the greatest E/R length ratios. End collars or flanges for mounting can be any size or shape, whether larger or smaller than the boot.
Stitched bellows cannot be completely sealed, so adherence to air-, light-, or dust-tight requirements cannot be guaranteed. Coating helps, but a sewn boot still tends to &#;breathe&#; through its seams. For areas where fine contaminants require more complete exclusion, it may be necessary to seal the boot so tightly that it may collapse if the travel speed is too rapid. Realize that the boot is actually a bellows, so a vacuum is generated during rod extension, and internal pressure develops during retraction if adequate venting is not provided. Methods will be described for providing the necessary makeup air while still excluding contaminants.
For more complete sealing and improved appearance, a vulcanized bellows of Hypalon-coated polyester material can be used. These are similar to the stitched bellows, except that seams are vulcanized instead of stitched, making them air-, light- and dust-tight. Because these seams are wider, the E/R ratio is slightly lower than with standard stitched construction. This factor becomes critical primarily if space for the boot is limited when the cylinder rod is retracted.
Injection-molded bellows configurations offer complete sealing but present some disadvantages. Molds are expensive, so volume requirements for cost effectiveness may be too high for most applications. Because the material is not reinforced, it lacks strength and can tear; and the rounded convolution shapes usually exhibit a poor E/R ratio. In most cases, a suitable custom boot can be produced by other processes using standard tooling and components.
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An excellent alternative is a bellows- type cover expanded from a tube of fabric-reinforced elastomer. It provides the same tight-sealing advantages of a molded boot, but with additional strength and low tooling charges, if any. These boots are cured or vulcanized after forming and can be made with a variety of end mounting configurations. Ranges of E/R ratios and stroke-to-retracted- length (S/R) ratios for the various designs and material thicknesses are available from manufacturers.
Many different materials can be used to produce rod boots, depending on the operating conditions and the manufacturing process selected to meet the needs of the application. Base fabrics include nylon, fiberglass, and Kevlar. Elastomers commonly used for boots include butyl, nitrile, neoprene, silicone, synthetic rubber, fluoroelastomer, PTFE, and polyurethane. Other specialized materials also are available, such as aluminized fiberglass for welding applications.
Steering rack boots/bellows are used to protect the inner steering tie rod joints and the steering rack ends and shafts from road debris and water splash. The boots are made from flexible plastic or rubber, which allows them to flex in and out as the steering rack pushes and pulls on both sides of the steering linkage to steer the vehicle.
The rack boot is a protective component. This means that boot failure can result in the failure of the components that it&#;s supposed to protect. In this case, the protected components (inner tie rod and steering rack) are both steering system components. Failure of either of these components will cause a safety concern.
A small tear in the boot, if caught early, probably has not allowed much debris intrusion yet. The odds are good that debris intrusion has not yet caused the inner tie rod steering joint to wear. It&#;s also unlikely that any significant debris has intruded onto the rack shaft or accumulated in the rack end. Proactive boot replacement has a good shot at preventing a more costly failure.
The longer a torn boot remains in service, and the larger the tear in the boot, the greater the odds of inner tie rod joint failure or steering rack trouble.
Debris and water can intrude into the inner tie rod steering joints through a torn rack boot. This can result in debris loading of the joint and lubricant wash-out. Inner tie rods are steering linkage components. The steering rack pushes and pulls on these joints to steer the wheels. Loose tie rod joints can affect the vehicle&#;s stability, steering responsiveness, and alignment. A severely stressed or worn tie rod joint may separate completely, which is a very dangerous scenario (this results in complete loss of steering control for one wheel).
Debris will intrude through a torn rack boot. If debris finds its way onto the steering rack shaft, it may get pulled back into the rack and jammed into the shaft seal. For example, a tight left turn will result in the rack shaft extending out on the right hand side of the rack. This could expose part of the shaft to debris. When the wheels are then turned back the other way, this shaft, now carrying debris, retracts back into the rack and toward the shaft seal. If the shaft seal is torn by debris, this will result in power steering fluid leaking from the rack.
OE grade
Generally speaking, the inner tie rod joint should be checked for play before boot replacement is performed. This helps account for the possibility of debris intrusion or lube wash-out resulting in wear, and sometimes these joints wear out anyway even if the rack boot is intact. Joint replacement will require rack boot removal anyway so we don&#;t want to go installing a new boot over a worn joint.
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