It is not necessary for the body to be raised like in standard dump truck, where the load exits through the rear. Instead, the bottom dump unit is positioned in place and trap doors at the low bottom point open up to release the freight. Gravity does the rest.
The bottom dump trailer, as we know it today, first took its current shape in the early 1970s. Although the principle of bottom discharging can be found long before that – for instance in wheelbarrows, horse drawn carts or, most prominently, rail wagons – they all served to generate the idea of designing an articulated trailer that could be pulled by a truck. In the US, the design is still referred to as a ‘hopper’.
The attraction to bottom discharge was and still is the simplicity of mechanically opening a discharge gate as opposed to designing an end dumping solution, which, at the time, required hoists, power packs and a flat stable ground to discharge onto.
Today, the bottom dump concept is still used in the construction industry where it originally evolved from, but also to cart bulk commodities such as grain, aggregate and asphalt on-road. The size of a modern hopper is naturally governed by local weight limits, but there are three general categories around the globe, depending on gross capacity – up to 25 m3 for on-highway applications, 30 to 40 m3 for on/off highway use as well as 40 m3 and more for off-highway use.
On the design front, the product to be carried dictates the hopper’s shape and size as all products have a particular density and angle of repose. Density usually decides the volume of the tray, while the angle of repose decides the slope of the sidewalls to enable free flow and complete discharge of the product.
A bottom dump trailer is usually constructed from steel or aluminium, although sophisticated designs have appeared for the transportation of hot asphalt, which requires insulated walls to keep the product warm and free flowing.
To connect the hopper to the suspension and skid plate, designers generally used a space frame made from square tube that bridges the three trailer elements into one. A different approach is the double skinned solution, one where a full-sided exterior is fitted out internally with hoppers, as US company Wilson demonstrated.
Recently we have seen some variations of that approach, especially in Australia. Local company, Graham Lusty, has been using the outer shell of an end dumper model and then added hoppers within the outer shell.
The discharge of a bottom dump trailer can be transverse or longitudinal to suit the application required. The transverse type is typically used in construction where aggregate for material such as crushed rock and gravel is best dumped as wide as possible to reduce the levelling work required by other machinery. The longitudinal type, meanwhile, allows the product to be dumped and reduce damage caused by wheels travelling over the product. Also, it is used over underground conveyors, which then take the product to silos, for example.
There are also bottom dump units that have more than one hopper compartment and those that have more than one discharge station, which can be transverse or longitudinal.
The trap door of the discharge, often called gate or clam, usually is the crux of a bottom dump’s performance. Manufacturers have invested an immense amount of time perfecting their designs to ensure the most efficient result. However, the design of a door is not simple and often limited to a specific cargo type only. While some doors can open completely, some are designed to open partially only – but they all need to seal completely to ensure no product is lost in transit. The doors are typically activated by pneumatic or hydraulic cylinders and activated by remote or in cab controllers.
To cover the load, there are four basic types of tarps available. Firstly, the roll-up type, which is rolled onto a drum and stored at one end of the hopper. To cover the load, it is pulled longitudinally to the other end. Secondly, the lever arm type, which is a mechanical aid to unrolling the roll-up type. The arm is pivoted at the centre of the body and powered to pull the tarp to the other end of the body. The third type is the slider tarp, which bunches up at the truck end of the body and then extends to the other end of the body. The fourth type is a transverse roll-up tarp, which can be manually wound from the rear of the trailer to be opened or closed. Electrical versions are also available.
One key market of the bottom dump design is North America, where it is widely spread in the agricultural industry. The most prominent names in the US are Manac and Timpte, who are now selling into the Australian market as well. The 130-year-old company is considered the leading force in the North American grain hopper market and has recently unveiled a multi-purpose model with a removable auger system that can unload 26 tonnes of seed or fertilizer in minutes, according to Timpte.
Manac, meanwhile, own the relatively young CPS brand, which is focusing on bottom dump technology only, mainly for the agricultural industry and with a strong focus on lightweight design. But the competition is tough, especially in North America, where Beall, Clement, DeLucio, Load King, Load Line, Ranco, Western and Wilson are also competing for business as well. Wilson, for instance, has gained renown for the aluminium-based Pacesetter model but also offer a sloped double-wall model and a B-double variant. The MC series designed by Canadian contestant Midland, meanwhile, can boast an oscillating upper fifth to simplify discharging and also offer a B-double version.
In Europe, the market is also quite dispersed, with small to medium-sized companies like German silo-specialist Heitling and Feldbinder or Italian family company O.ME.P.S taking the lead in the bottom dump segment. In fact, O.ME.P.S has recently entered a co-operation agreement with Australian tanker specialist Tieman, who offer a bottom dump model specifically designed for the Australian market, competing with local brands like Barker and Graham Lusty, to name a few.
In the Middle East, a new player recently emerged in the Sultanate of Oman. Since 2008, Oryx Metal Industries has become one of the major players in the Gulf region, Africa and the Middle-East. The young company is specialised in the design, development, and assembly of various semi-trailers and rigid bodies, but also builds hopper-style bottom dump models. The Asian market, however, is dominated by global powerhouse, CIMC, who also have a stainless steel bottom dump model on offer all around Asia, in Australia, as well as in Europe.
Regardless of the origin, the bottom dump concept has proven that it is more than just a high-tech sinkhole on wheels. In fact, it can boast a range of advantages over a tipping solution, depending on the application, of course. Since the body does not elevate, a bottom dump trailer is usually more stable, eliminating any danger of the body tipping over flat on its side. The risk of contact with power lines or high overhead obstacles is also reduced, and there is more control afforded to the discharging process per se.