carried on its own three wheels Mounted or integramost use a three-point hitch and have a rear wheel in use only when ploughing. Some also have a gauge wheel to regulate maximum depth. Semi-mountedused principally for larger ploughs. These have a rear wheel which usually carries weight and side thrust when ploughing and sometimes the weight of the rear end of the plough when lifted. The front end of the plough is carried on the tractor lower or draft links. Plough wheel The gauge wheel is an auxiliary wheel to maintain uniform depths of ploughing in various soil conditions. It is usually placed in a hanging position. The land wheel of the plough runs on the ploughed land. The front or rear furrow wheel of the plough runs in the furrow. Plough protective devices When a plough hits a rock or other solid obstruction, serious damage may result unless the plough is equipped with some safety device. The damage may be bent or broken shares, bent standards, beams or braces. The three basic types of safety devices used on mould-board ploughs are a spring release device in the plough drawbar, a trip beam construction on each bottom, and an automatic reset design on each bottom. The spring release was used in the past almost universally on trailing-type ploughs with one to three or four bottoms. It is not practical on larger ploughs. When an obstruction is encountered, the spring release mechanism in the hitch permits the plough to uncouple from the tractor. When a hydraulic lift is used on the plough, the hydraulic hoses will also usually uncouple automatically when the plough uncouples. Most plough makers offer an automatic reset system for tough conditions or rocky soils. The re-set mechanism allows each body to move rearward and upward to pass without damage over obstacles such as rocks hidden below soil surface. A heavy leaf or coil-spring mechanism that holds the body in its working position under normal conditions resets the plough after the obstruction is passed. Another type of auto-reset mechanism uses an oil (hydraulic) and gas accumulator. Shock loads cause the oil to compress the gas. When the gas expands again, the leg returns to its working ploughing position after passing over the obstacle. The simplest mechanism is a breaking (shear) bolt that needs replacement. Shear bolts that break when a plough body hits an obstruction are a cheaper