All set for a good life
Summary
Ownership and mileage
New trucks:
Sold after three to four years or 1.3 million kilometres of service
New trailers:
Sold after about seven years or 1.5 to 2.4 million kilometres of service
On-the-road truck duty:
Most trucks accumulate about 400,000 kilometres a year
(Source: Hare Express)
Cost and savings
Based upon an hourly shop rate of US$50 and a truck down time cost of $50 an hour during maintenance, as estimated by Hare Express.
Normal brake inspection:
Savings per truck hub unit: one hour
Normal bearing service:
Savings per wheel: two hours
Truck hub unit in use for 800,000 kilometres (2 years).:
Service time reduction: three hours Downtime reduction: three hours Cost savings: $300 per wheel end or $1,800 per truck with total of six truck hub units.
SKF Truck Hub Units are specified integral components of the Meritor Easy-Steer steering axle and the TB series trailer axles at the time of new truck purchase.
(Source: Hare Express)
Behind the engineering scenes
Conventional bearing arrangements do not meet today’s critical requirements for a trouble-and-service-free long life. Errors during maintenance, service, replacement and adjustment are all too common. By design, it is virtually impossible for such errors to occur with the SKF Truck Hub Unit.
Bearings in the SKF THU are of a double-row, taper roller design aimed exclusively at heavy-duty truck applications. Specially engineered raceway profiles ensure optimum use of the contact area under a variety of load conditions, avoiding dangerous edge stresses during cornering and hard braking. Exceptionally tight control of operating clearances during manufacturing and factory assembly assure a degree of reliability significantly higher than that of both conventional and spacer-design hubs.
The SKF THU is also ideal for use with antilock braking systems. Its compact design leaves more room for the toothed ABS wheel and pick-up. ABS components can be mounted directly and precisely on the THU, improving the quality and accuracy of the ABS system. SKF Truck Hub Units were introduced to the European market in 1987, but it is only since 1995 that they have become available to the U.S. market. Market-based distinctions exist between U. S. and European truck hub unit models.
In the European market, some seven different wheels are used. Differences include lug bolt patterns, mounting designs and use of disk brakes. These differences require that European units be manufactured in multiple styles with different flanges to accommodate a wide variety of wheel-end configurations. The U.S. market, however, consists of heavy-duty trucks with drum brakes and single, standardised wheels.
Truck hub units have the potential to keep trucks on the road longer, with less downtime. Fleet operator Hare Express decided to try them out.
With the development of an integrated hub bearing arrangement that can be used on trucks and trailers, engineers have borrowed a feature from passenger car technology.
A hub unit combines bearings and hub in a factory-manufactured assembly that is lubricated, sealed and adjusted for life. Many passenger cars use such an arrangement. Passenger car wheel bearings are critical automotive axle components that experience a wide range of loads and operating conditions. Heavy-duty trucks (over-the-road tractor-trailers), however, experience even more severe usage in terms of considerably higher vehicle road time, consistent long-distance use, inordinately high bearing loads and frequent maintenance.
Maintenance of the conventional bearing arrangement, with separate bearings, seals and hubs, has long been a serious concern for both owner-operators and truck fleet managers.
At Hare Express, a fleet operation near Detroit, Michigan, in the United States, trucks normally accumulate 400,000 kilometres per year. Company trucks with conventional bearing arrangements are brought in every 160,000 kilometres as a matter of routine bearing maintenance, says Matt Hare, chief operating officer of Hare Express. However, Hare Express has recently added 20 tractors equipped with Meritor Easy-Steer Axles with SKF Truck Hub Units. Says Hare, “So far, we’ve exceeded 300,000 miles (480,000 kilometres) on these units – with no bearing-related problems and zero maintenance.
Truck wheel end maintenance A considerable number of European units have been in use for 10 years and continue in trouble-free service even today. The conventional arrangement of separate, non-integrated bearings, seals and lubricant are maintenance-intensive by nature, making maintenance downtime a fact of life for truck owners. Conventional truck hub bearings require periodic inspections and end play adjustments. Seals often fail well before their projected lifespans.
Bearing experts believe that more than 95 percent of all wheel-end bearing failures have occurred because they have failed prematurely due to contamination or poor adjustment rather than because they have reached the end of their fatigue life. Seal failure and subsequent oil or grease leakage starves wheel bearings of lubricant, causing excessive operating temperatures and premature bearing failure. Seal failure alone often necessitates expensive bearing and brake lining replacement. When the hub lubricant escapes past the seal, the brake lines are often contaminated.
Most hub seal failures are due to in-the-field maintenance problems including improper seal installation, inaccurate bearing adjustments and contaminated lubricant. Frequent bearing and seal maintenance – and downtime – is required to forestall failure of conventional wheel-end components.
The truck hub unit solution
Trailer axle maintenance occurs less often and is more difficult to monitor, in terms of accurate, accumulated road time, than other wheel-end positions. Trailer bearings meet the worst conditions possible for oxidation, condensation and other damaging conditions, having to endure periods of prolonged downtime in freight and other yards.
The primary objective of truck owners and fleet managers is to eliminate downtime and its considerable costs. A truck or trailer that’s down for any service task – including regular wheel-end maintenance – makes no money for its owners. Truck hub units address this critical issue head on, eliminating wheel-end maintenance and reducing downtime.
Both experts and truck owners agree that evolutionary designs, those involving slight modifications to present wheel-end designs, are not acceptable over the long haul. To really prevent wheel seal leaks revolutionary designs are needed.
Instead of using separately installed, replaced and adjusted items for the wheel end, the truck hub unit is fully integrated – factory assembled and adjusted for life – eliminating the need for maintenance of any kind. In the unlikely event that a unit is damaged in service, i.e. by accident, the entire unit can be replaced as a single assembly in less than 30 minutes, with no in-the-field adjustment or lubrication required.
“Our trucks use external hubs, so we don’t even remove the truck hub units during brake lining replacements,” says Hare. “People are generally sceptical of the term ‘zero maintenance,’ but truck hub units have proven themselves even to our own sceptics.”
Stephen J. Davis
a business writer based in Philadelphia, Pennsylvania, USA
photo Dave Gusho
