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RATIONALIZING THE CHCIICE
Page 76
Page 77
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Producing suitable gears for all applications is no problem but building a matched combination of ram assembly, body and chassis at the right price and weight is a job for the specialist.
by Paul Brockington, MIMechE MATCHING a tipping gear and body to the vehicle chassis has over the years become an exact and exacting science that has been applied to a worthwhile extent to the majority of newer tippers on the road. But the advent of the bigger vehicle and its exploitation to carry the optimum payload, together with the prospect of 38/44-ton gross vehicle weights are posing new problems related to stability on rough site work and weight distribution.
Whether a singleor twin-ram front-ofbody gear is the most suitable for longer vehicles, is currently the most controversial of tipper topics and it is noteworthy that some makers say that they only fit twin-ram gears to satisfy the preference (or prejudice) of the operator. While the single-ram gear is often cited as the preferable unit if the body lacks torsional rigidity (because it obviates differential action), Edbro emphasizes that the use of a single ram does not eliminate overstressing if body flexing is excessive and that a torsionally stiff chassis is always desirable. Other advantages of the single-ram gear mentioned by Edbro include a reduction in the number of seals that are liable to leak and in the size of the recess in the front of the body if a recess is needed. The disadvantages given include a higher cost in some cases on account of the greater tube thickness required, greater concentration of ram thrust and the increased space required at base level in the fore-and-aft plane. In some applications the relatively flexible twin rams are the more suitable.
The problems of stability are magnified -f there is an increase in g.v.w. that is not • nmpanied by an increase in overall base width. There is no lifting problem in catering for maximum gross weights, the Edbro 8EN twin-ram gear having been employed to lift 34ft bodies in the case of Continental applications. Employing a body of the correct length relative to the plated load of the vehicle is cited by Edbro and other makers as the all-important factor in the avoidance of axle overloading.
In a detailed review of matching problems associated with the production of long tippers with custom-built bodies, Edbro includes a "reasonable" tipping frequency of 10/15 tips a day in an outline of standard operating conditions for which the recommended gears are suitable and which also include uniformly, distributed loads of "ordinary" excavated materials and of coal, sand and 'batching materials. For a vehicle used to haul sticky materials in arduous conditions, such as the transport of excavated material that has been loaded unevenly by a power shovel, de-rating of the tipping gear is advocated or application of a gear of higher capacity is recommended. A number of applications to vehicles produced overseas are in this category.
Undesirable features The added cost and complexity of installation created by the features of modern vehicles are cause for common complaint among tipping-gear makers generally and the Edbro list of undesirable features is typical. This includes the protrusion of engines and location of air cleaners behind the cab, tapering frames in this area, cluttering of the space in the frame area with air tanks, servos and so on, and in some instances the use of gearboxes that require a special adaptor to take a standard pto. While the associated problems can be overcome, the special units required add to cost and this could influence the operator in his choice of a chassis. With an eye on the future, Edbro emphasizes that continuous research is necessary to anticipate and provide for new developments.
Twin-ram front-of-body gears have no advantages over the single-ram type, according to Telehoist technicians, and the single ram gear is the more stable, although stability should whenever possible be the subject of a tilt test. They point to the single ram of the company's Hy-Spill tipping trailer as an example to back this statement. The ram is a five-stage gear with a lifting capacity of 35 tons and it is notable that the fixed-sided tipping body is an extremely robust structure made of BS 4360 high-tensile steel.
Although Telehoist says that mismatching of tipping gear, body and chassis is now relatively rare, and that the design standard of the majority of combinations is far higher than it was a few years ago, the company's engineers are in agreement with other makers that nearly every chassis poses an individual set of problems. This is particularly so because of variations in main-frame-niember design, and wheelbase, power unit options and fuel-tank mounting. In the case of a 16-ton tipper, the difference in weight or two optional power units may represent a difference in centre of gravity longitudinally as much as 2in.
Anthony Hoists is confident that the rigid tipper coupled to a draw-bar tipping trailer has an important future if the driveronly prospect materializes. Such an outfit will have a very important future if a g.c.w. of 50/56 tons iS eventually permitted. The company has bUilt artic tippers for export —with Anthony/Autolifts single-rain gears —capable of carrying payloads of up to 40 tons in a 2811 body length. It is expected that in line with Continental practice, combinations having a draw-bar trailer would be of the side-tipping or three-way-tipping type. The ability of the tractor unit to operate on its own would provide valuable flexibility, and the individual parts would be more stable than the artic tipper on rough sites. The reversing difficulty could be a big disadvantage, hOwever.
Single-ram gear
Anthony Hoists emphasizes that the single-ram gear has "all the advantages" • compared with an equivalent twin-ram gear including a greater resistance to bending, which is cited as the main cause of failure of tipping gears. The company also points out that no twin-ram gear is made in the UK having a flow-dividing valve and that absence of such a valve is the cause of differential action resulting from the disproportionate amount of fluid that flows into the ram with the smallest load.
A Welford Engineering spokesman conskiers that the single-ram front-of-body gear has yet to prove itself in service in terms of wear and maintenance costs, and that the twin-ram may come back into its own despite the fact that a "lazy ram" can produce differential action. Welford-2-makes rams in small quantities for applications requiring a design that cannot readily be supplied by the bigger tipping-gear makers (at a lower cost) but production is threatened by the non-availability of suitable tubes in
small batches. While the company's spokedman paid tribute to these makers for the excellence and low cost of their equipment, he said that the equipment recommended for an application frequently has a capacity that is greater than the loading in service, and that often a vehicle was "over-rammed". He claimed that a large number of unsatisfactory tippers are being produced by less reputable body builders.
To Hanger Engineering the matching of the ram gear to the chassis and body of Eagle municipal vehicles is a case of matching the life expectancy of the gear to the anticipated life of the vehicle. This is normally at least 10 years and may be as much as 30 years according to type. Displacement type tipping gears of Hanger design and manufacture are fitted to heavier Eagle municipal vehicles but relatively low-cost proprietary rams are in the main used for the conventional tipping vehicles built at the company's Always Works in Aldershot, and for Eagle side-tipping refuse collectors. Producing ram equipment in the workshops ensures that the high standard of know-how required is perpetuated.
Moreover, the tipping ram gear is often a relatively small part of a complicated hydraulic system which includes rams for other purposes of special design which could not be bought out and which require a highcapacity oil reservoir. Underbody gear is not favoured for refuse or tipping vehicles and a single-ram front-of-body gear combined with a stabilizer is preferred to a twin-ram type.
Technicians of Milshaw are in favour of fitting stabilizers (of the rack-and-pinion type) to all types of tipping vehicle and of using a big single front gear in place of twin rams for long vehicles. They point out that a single ram equipped with a gimbal headboard gear is not strained if the body twists, whereas twinned rams are distorted by twist and are liable to lock. Tortional rigidity of the body is of less importance if a stabilizer is fitted.
Tipping load indicator The company, produces a tipping load indicator and it is forecast that tipping and/ or axle-load indicators will play an increasingly important part in tipper operations. Strict enforcement of the weight regulations would facilitate the job of the bodybuilder in providing a matched combination. The company specializes in underbody gear and exports gears of this type to the Continent in quantity.
Although Spenborough claims that the robust cross-head of the sling-type lifting frame fitted to vehicles with twin front-ofbody gears eliminates differential action, it is significant that a heavy-duty single-ram gear is strongly recommended for heavier chassis. While the thrust of the 25-ton single ram on a semi-trailer with a 25ft boo./ is transmitted to the underside of the body at the front, the single-ram gears of smaller rigid vehicles are connected to a compensating cross-head fitted to the sling frame which picks up on extensions of the longitudinal body. Well-proven in service, the sling-frame method of body elevation facilitates the use of a body designed solely as a load carrier because the cross-head and frame assembly prevents body distortion and relieves the ram tubes of side strain.