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MAKING
TRACKS
By - Hank LeMeur {Company President}
IVT Inernational, February 2002
The growing popularity of rubber-tracked vehicles - using either friction-drive or positive-drive systems - belies the critical design
considerations that ensure smooth performance
Rubber tracks for industrial vehicles have gained popularity in the last decade in a range of applications. Performance requirements differ by application, but
rubber track undercarriage systems are very versatile, as the following examples demonstrate.
Two benefits for agricultural tractors are lower ground pressure - resulting in lower soil compaction for an increase in crop yields - and greater pulling force
which increases plowing throughput. Skid-steers take advantage of increased flotation for better traction in muddy conditions while avoiding damaging pavement
surfaces. Man-lifts use track systems for better stability to avoid tipping in outdoor use, while seismic survey equipment uses the lower
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ground pressure tracks to avoid damaging sensitive environmental areas, while also having the flexibility to climb over rough terrain. Asphalt equipment uses
rubber tracks to ensure a smoother ride on a rough surface, resulting in flatter paving. And all applications benefit from avoiding pneumatic tire punctures.
Critical considerations
But that is not to say that rubber track systems are without technical challenges. There are critical system design considerations and component selection criteria
to ensure rubber track systems perform as expected.
Figures 1 and 2 demonstrate the basic elements of a friction-drive system and a positive-drive system. As the name
implies, a friction-drive system relies on the contact between the drive wheel and the track to propel the vehicle. The positive-drive relies on the cont act between a pin
on the drive wheel (sprocket) and a lug (chain) on the track for propulsion.
Both friction-drive and positive-drive systems have the same basic components: drive wheels as described,
idler wheel(s) which provide belt tensioning and tracking,
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bogie wheels which distribute the load, the rubber belt and possibly a suspension system. (The term track refers to the complete system and belt refers to only the rubber band).
The wheel assemblies provide a channel for the belt lugs to travel through, ensuring that the belt stays on the unit. Heat generation: Heat generation is a critical design
consideration in any system. In rubber track systems, it is a leading cause of premature component failure. There are many potential sources of excess heat and the rubber components'
physical properties have a relatively low tolerance of heat build-up. Figure 3 plots two important physical properties of rubber versus temperature. Tear energy - a measure of material
toughness important to the belt and tires - decreases from an ambient 30°C to 100°C. Rebound - which correlates with abrasion resistance - begins to drop off above 90°C. Manufacturers
have proprietary compounds and continually improve the physical properties of their materials, but the graphs shown are representative of the effect heat.
In track systems, most heat is frictionally generated by rubber-to-rubber sliding contact and rubber-to-steel sliding contact. As discussed in greater detail
under component design, the objective is to minimize sliding contact between
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components and to reduce the coefficient of friction of the materials used. Sliding contact can be minimized through
wheel design, belt design, load equalization through suspension, and bogie wheel size and placement.
Material selection for undercarriage and belt components needs to be carefully considered, not only to use the lowest acceptable coefficient of friction materials, but also to
select
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| mutually compatible materials. For example, two components constructed of materials of equal hardness and the same coefficient of friction will generate more heat than two
components of dissimilar materials and a higher and lower coefficient of friction. This is due to the dynamic of the contact interface.
Debris ingestion: Debris trapped between the belt and the idler or
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drive wheel exerts large forces due to the belt tension. This point loading of debris can damage the belt and/or the wheel. Ingestion between the drive wheel
and the belt can be minimized by placing the drive wheel above the ground (Figure 2). Shields to keep debris out are not widely used and can create problems by
trapping debris, while scrapers used to clean belts have not been successfully implemented.
Belt tensioning: Tensioning is required in friction-drive systems and positive-drive systems because the contact between the wheels and the belt ensures that the belt stays
on the track. While a mechanical jack screw might be used in low- cost designs, the preferred tensioning systems uses hydraulics with a relief system. The relief system is
necessary to minimize damage from ingested material. With no relief, point loading may cause damage to the wheel or belt.
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The required tension force needed by a positive-drive system is approximated using about 40% of the vehicle weight that is supported by a single track.
For example, a 60,000-lb unit would require 12,000 lb of belt tension per track. An idler wheel as the hydraulic tensioning member is constrained by two sides of the belt. So for
this example, a hydraulic force of 24,000 lb is needed to load the idler wheel. A friction-drive system may require about 50% greater tension that a positive-drive system. Clearly
the undercarriage needs to be robust.
How does the track handle these loads? General-purpose belts have an allowable design tensile load of about 1,000 lb per inch of width (PIW). For the case discussed, a 30in wide
belt would withstand a load of 12,000 lb/ 30in, or 400 PIW - well within the design limit. The ultimate fatigue limit of the belt is determined by the imbedded steel cord. An ultimate
tensile strength
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approaching 10,000 PIW is typical depending upon the belt and manufacturer.
To properly support these high tension levels, both the idler and drive wheels need to fully extend from the lug at the center of the track to within un of the edge of the belt.
The 24,000-lb load on the idler wheel might seem excessive, but this is not the case. For the track discussed above, assume that 20in of width are available for wheel contact.
Assuming a 120° wrap as shown in Figure 2 and a 30in outer diameter, the total contact area is 628in2, resulting in a tire loading of 38psi, excluding the
vehicle weight and load. This loading level, in addition to any vehicle loading, is well below the 150psi rubber tire loading limit.
Finally, the hydraulic tensioning system needs to accommodate belt stretching of about 1.5%. This means that the tensioning device
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| on a 30ft long, large track system, would need to extend ±5in from nominal. These are approximate examples and exact tensioning levels
need to be determined during the prototype testing phase.
Belt tracking: Belt tracking describes how the belt stays within the channels created by the halves of the drive, idler and bogie wheels. When turning, or on sloped
terrain, the guide lug of the belt will contact the sidewalls of the drive, idler and bogie wheels. This is necessary for the function of the unit but it is critical to
avoid built-in (either by design or manufacture) lack of parallelism or a cant that will cause continuous contact between the lug and wheel. Poor tracking will result in
heat build-up and possibly the belt jumping out of the wheels.
In addition to building the tracks 'square', 2in spacing of lateral clearance between the wheels and belt lug is recommended. Designers must test their units in the
design phase and in product ion by monitoring track temperature while the unit is being tested. Rapid
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increases in tire and belt temperature indicate an alignment problem. There are several successful designs with means to adjust the alignment.
This is important in applications where there are large lateral loads, for example, when a machine is expected to traverse sloping terrain.
Components selection criteria
The two major manufacturers of large equipment belts are Goodyear and Bridgestone. The cost for the rubber belt can range from US$1,000 to US$16,000, depending upon
quantity and size - which can range from 7-36in wide and up to 31 ft long. Both manufacturers use a similar basic belt structure. A representative cross-section of an
excavator belt is shown in Figure 4.
There are three elements to a rubber belt - the carcass with steel for longitudinal strength, the tread with rubber of high abrasion and cut
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and tear properties for wear, and the guide lug, which can be made of steel or a high modulus rubber material (for higher performance belts), that preferably has low coefficient
of friction. The pitch of the track is the distance between repetitions. It is critical that the pitch matches the drive wheel outer diameter. The proper mating of these
two elements minimizes sliding friction between the belt guide lug and the drive wheel pin.
The key design variable in the tread is the net to gross contact area. Agricultural belts have an aggressive tread to maximize traction with a net to gross contact area
of 20-40%, whereas a construction tread has 50-70%. The construction tread contact area is much larger because it results in slower wear on abrasive surfaces. Spacing and
orientation of the belt's lugs with relationship to the bogie wheels is critical to minimize vibration while running. For example, Figure 5 shows how bogie wheels could
synchronously drop into the belt tread lugs causing vibration.
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Drive wheel: As the name suggests, this imparts the torque from the engine to the belt.
To maximize the power of a drive unit, the designer wants the highest number of lugs in contact with the drive wheel or the greatest angle of wrap of the belt around the wheel,
with the largest radius available. The bars on the drive wheel that contact the belt lugs need to be as smooth as possible. If solid, the welding holding the bars needs to be well
ground. It is also advisable to avoid pinch points that may cause the rubber belt to tear. For units that operate at higher speeds, the drive bars should have sleeves that can rotate.
These will minimize sliding friction between the bar and the lug.
The pressure between the belt and the idler and drive wheels imparted by the tensioning unit was discussed earlier. One might think that a high friction material would be
preferential to aid the bars in driving the belt. This is not the case, as excessive friction on the drive wheel can interfere with the operation of the lugs. For all track system
wheels, it is best to extend the wheel to within an inch of the edge of the belt to ensure there is no differential longitudinal stress on the belt.
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In a low-speed application with limited exposure to debris, there may be no need to cover the drive wheel with a rubber cover. However, in high-speed/high-debris
applications, a cover of 0.5- 1 .5in of rubber will improve belt work- life as the rubber wheel cover will absorb some of the point loading caused by ingested debris.
Bogie wheels: In general, due to geometry constraints and the need to provide as many contact points as practical, bogie wheels are approximately one-third of the diameter of
drive wheels. Consequently, they rotate three times as fast. It helps to view a wheel as a heat pump with each rotation resulting in a compression cycle at a given point on the
wheel. Three times the rotation means three times as much energy input to the bogie versus the other wheels. The consequence is that the bogie wheel heat management is paramount
for the life of the wheel and (due to contact with the track lug) life of the track lug. The goal of wheel life should be 9,000 hours. Unlike industrial tires, the bogie tread
surface will not experience wear. If there is no debris damage and heat generation is managed properly, extended wheel life should be achievable. Wheel failure is due to abrasion
on the track lug and premature heat failure.
It is critical that the profile of the bogie wheel (and idler wheel) sidewall match the profile of the track lug so that excessive compression does not generate heat. Various
belt manufacturers recommend a radius on the wheel
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not less than 2in. Figure 6 shows that the wheel sidewall matched the lug profile and that there is a gap between the radius of the bogie and lug. This
gap allows the rubber to expand under compression so that there will not be a pinch point resulting in excessive heat. The wheel manufacturer also wants the rubber sidewall to be
as smooth as possible and to avoid metal edges that will become exposed as the rubber wears.
Various implementations have used rubber or other elastomers as the cont act surface with the belt lug. Here, the rubber extends around the lip of the wheel, covering the contact
area. This should only be done when the rubber has a low coefficient of friction.
Given that the interface between the wheel and the lug is designed properly, selecting a slightly harder wheel rubber with a higher coefficient of friction will minimize heat generation.
A good choice is Superior's 85-90 Shore A hardness polyurethane. The typical hardness of a rubber track belt is 75A. The difference in hardness between the bogie wheel and the lug will reduce
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| heat gene rating scrubbing action. In addition, polyurethane has an intrinsic coefficient of friction of 0.6 relative to a rubber coefficient of 1.0. Additives to this can
reduce the coefficient further.
One relatively complicated mechanical means to minimize frictional heating patented by Caterpillar is to use an independently rotatable shield to contact the track lug. The advantage of this approach
is that the relative velocity between the bogie wheel and the track is minimized. This relative motion is the source of sliding friction. Another approach is to use heat shields. These can be constructed
on bogie wheels that dissipate heat by designing the area that contacts the belt lug to be thermally isolated from the wheel rubber by using mechanical means, oil or seals.
Idler wheels: The bogie wheel material selection and contour guidelines above also apply to idler wheels. A difference is that the idler wheel is typically
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larger than the bogie wheel reducing the energy load. The size of the idler wheel is relatively large so that radius about which the belt turns is not too small. A tight turn
radius puts large tensile stresses on the surface of the tread, which makes the tread more susceptible to cuts and tears.
Suspension: Pneumatic-tired equipment has built-in cushioning. This is at the cost of excessive bouncing under certain speed and terrain conditions. Simple rubber track systems with
no suspension can be rigid, resulting in poor ride, conditions of uneven loading and poor obstacle clearance. A suspension can be expensive but for applications on undulated surfaces,
it may be needed. Suspension systems need to accommodate upward and rearward motion. There are many solutions to the challenge - a trailing arm configuration with an elastomeric or air
spring is most often seen in practice. Such a system must maintain relative spacing of the drive and idler wheels while
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accommodating the location of the hydraulic tensioning system.
Track and field
Track systems have a decade of extensive field performance and improvement behind them. During that time, performance has increased from a speed of feet per minute to over 30mph,
vehicle weight from hundreds of pounds to 100,000 lb and track widths from 7-36in.
System improvement and development continues at a rapid pace. Many patents on the design considerations previously discussed have been awarded during the last several years and are
still under development. IVT
Article in PDF Format (15mb)
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