Gravel transfer trailers are also popular for moving aggregate Usually built out of aluminum for a higher payload. The truck dumps the trailer by winching the trailer box onto the truck. The truck box must be dumped first allowing loading of the trailer box. The trailer box is pulled onto the truck by means of a hydraulic chain drive. This process is also used in other trailers such as a live floor model. A hog fuel or sawdust trailer may be equipped with a chain drive floor allowing scrapping and removal of material. A high lift tail-gate on the truck box allows passage for the trailer box. Great care must be taken when transferring the trailer box to the truck. There is hooks that lock the trailer to the truck box. Faulty loading and locking has resulted in operators loosing the trailer box off the truck.
After a successful dump of the trailer box, it is then winched back onto the trailer frame. Operator locks down the trailer box and re-attaches the trailer hitch to the gravel truck.
The truck box and front trailer frame will usually have a big dowel connection to assist in correct chain drive position for transferring. There is various models and configurations of transfer trailers. Extra controls for the operator may allow of partial truck movement remotely. This is done by leaving truck in gear and bumping the starter. truck will move a few feet but not start. This aids in hooking and unhooking trailer to the pintle hook.
Unhooking the trailer allows the truck into tighter quarters than if pulling the trailer. Placement dumping and stockpiling are easy tasks for a transfer unit. Automated air or electric tarps contain fine material from blowing off the load. Air gauges aid in weight measurement if equipped with air bag suspension. Electronic pads can also be used for net weight calculation. They are prone to malfunction but work good in certain environments. Electronic scale pads have been used on logging trucks successfully for many years.
Construction companies may special order their gravel equipment with different applications in mind. If ride is not a factor they may prefer a low maintenance heavy duty suspension. Height is also a consideration when using a transfer trailer. The transfer box on the trailer usually sits higher than the truck. When in low clearance situations, this must be considered. Loads of all shapes and sizes have become stuck or wedged in under-passes,bridges and tunnels. Checking your height before finding out the unit does not fit. An extendable height bar can be easily attached to the truck or trailer taking the guess work out. An old trucker trick is to let the air out of the air ride suspension if air bag equipped. This can lower the unit several inches and allow extra clearance. Care must be taken to move slow as the drive line will not be in the correct operating alignment. Use little power and slow speed when suspension air is removed. If really tight for clearance, air can also be removed from tires. They must be inflated again before preceding further.
The term "transfer truck" is quite general and can be used to describe different types of transfer trailers. Refuse trucks transfer waste by means of hydraulic rams that pick up dumpsters and containers.