Truck Equipment Glossary - H

Helical Gears
Gears with slanted teeth, usually used in transmissions. The teeth are positioned diagonally across the face of the gear.  
 
Hoist
Mechanism for elevating bodies, either level or tilting.  
 
Hold-over Plates
Plate coils containing a refrigerant that are mounted on the walls of a truck or trailer. When "charged"; these plate coils hold the temperature down to the desired point for milk delivery and many other low-temperature trucking operations.
 
Horizontal Center of Gravity (HCG)
The point at which half of the gross weight is forward and half is aft. See Also Center of Gravity
 
Horsepower (Gross)
Obtained by dynamometer test of an engine with water pump(s) operating and air cleaner, but without generator, fan and muffler. 
 
Horsepower (Net)
Obtained by dynamometer test of a complete engine with all standard accessories including water pump(s), generators and fan operating, air cleaner and muffler.
 
Horsepower (Taxable)
The National Automobile Chamber of Commerce (NACC) adopted an arbitrary formula for estimating horsepower to enable comparison of engines on a uniform basis. It assumes that engines deliver their rated power at a piston speed of 1,000' per minute and that mechanical efficiency will average 75%. Advancements in engine design since this formula was developed have made the formula obsolete completely as a basis of estimating true engine output. However, the formula is still used in some states for licensing purposes.
 
Hotchkiss Drive
A term applied to that type of chassis design where the rear springs are mounted at the forward end in a stationary bracket (not shackled as at the read end) and all driving and braking forces are cushioned by the springs and transferred directly to the frame side members. Open-type universal joints and propeller shafts are used in this design.
 
Hotel Loads
Power demands (loads) placed on a vehicle to help maintain cab environmental conditions, such as heating, cooling, lighting and accessories (e.g., radios).
 
Hybrid (Work Truck)
The combination of two different drive systems (conventional and alternative) to increase efficiency and reduce fuel consumption. Types of hybrids include:

  • Electric Hybrid — A hybrid drive system that uses one or more electric motors, in parallel or series configuration, as the alternative drive system. Electric hybrids typically use batteries or ultracapacitors to store energy which is generated onboard the vehicle by the conventional engine and through regenerative braking. 
  • Hydraulic Hybrid — A hybrid drive system that uses one or more hydraulic motors, in parallel or series configuration, as the alternative drive system. Hydraulic hybrids typically use accumulators to store energy which is generated onboard the vehicle by the conventional engine and through regenerative braking. 
  • Hydraulic Launch Assist™ (HLA) — A hydraulic hybrid regenerative braking system introduced by Eaton Corporation. The HLA system recycles energy by converting kinetic energy into potential energy during deceleration via hydraulics, storing the energy at high pressure in an accumulator and returning it to the vehicle during subsequent acceleration. This process reduces the amount of work performed by the internal combustion engine. 
  • Parallel Hybrid — A hybrid drive designed so that either system can propel the vehicle independently or in unison. 
  • Plug-in Electric Hybrid (PHEV) — A variation of an electric hybrid that can charge the onboard energy storage system by plugging into the electric grid when the vehicle is not in use. Plug-in hybrids typically have greater onboard electric storage capacity than regular electric hybrids. 
  • Series Hybrid — A hybrid drive system designed so that only one system (normally the alternative) provides power to propel a vehicle. The conventional power source is used to provide energy to the alternative drive. 
Hydraulic Control
Valve A mechanical device to divert or control the flow of fluid in a hydraulic circuit.  
 
Hydraulic Hose
Flexible oil lines used to transmit fluid.
 
Hydraulic Oil
Fluid used to transmit power for operation for hydraulic systems.  
 
Hydraulic Relief Valve
A mechanical device used to limit the pressure in a hydraulic circuit.