A bleed screw is a device used to create a temporary opening in an otherwise closed hydraulic system, which facilitates the removal of air or another substance from the system by way of pressure and density differences.

Applications

Domestic heating radiators

On a home

radiator A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

unit, the bleed screw can be opened, usually by means of a key, to allow unwanted air to escape from the unit. Bleed screws are also found on some pump types fulfilling a similar purpose. They are most often located at the top of the radiator on the side of the inflow pipe. The screw itself, usually a hexagonal or square knob, is inside a small round protrusion. The key looks similar to that used to wind a clock. It is inserted into the protrusion, mates with the bleed screw and turns it. Opening the bleed screw then allows air which has risen to the top of the radiator system (the top of the radiator itself) to escape and new water to take its place. Removing the air and allowing water to displace it makes the radiator work more efficiently since water conducts heat better than air. A domestic radiator may need bleeding in this way either during a heating upgrade, power flushing or simply when the radiator feels colder at the top can indicate there is air in the radiator that needs to be released.

Engine cooling

Engine cooling Internal combustion engine cooling uses either air or liquid to remove the waste heat from an internal combustion engine. For small or special purpose engines, cooling using air from the atmosphere makes for a lightweight and relatively simple syst ...

systems can also have bleed screws. They can take the form of a bolt with a hole through the middle that is threaded into a hole on the engine's

cylinder head In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern ...

. This hole goes into the

water jacket A water jacket is a water-filled casing surrounding a device, typically a metal sheath having intake and outlet vents to allow water to be pumped through and circulated. The flow of water to an external heating or cooling device allows precise ...

of the

. In other designs, the bleed screw is placed in the uppermost hose which leads to the

heater core A heater core is a radiator (engine cooling), radiator-like device used in heating the cabin of a vehicle. Hot coolant from the vehicle's engine is passed through a winding tube of the core, a heat exchanger between coolant and cabin air. Fins att ...

, i.e. at the highest point of the cooling system. When the bleed screw is loosened,

antifreeze An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allow ...

is added to the engine's cooling system and the increase in fluid pressure displaces air through the opened bleed screw. When liquid begins to flow out, all air has been removed from the system and the bleed screw is closed. Bleed screws are not common on cars today and are only necessary when design of an engine's cooling system results in areas where air can be trapped in the system. Air in the system can lead to overheating of the engine and in modern cars also to poor vehicle operation (e.g. problems with the AC system or incorrect

engine idle An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

Hydraulic braking systems

On a hydraulic vehicle braking systems, the bleed screws (sometimes known as ''bleed nipples'' or ''bleeder valves'') are located at the top of each

brake caliper A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc (sometimes called a rakerotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive friction and adherent frict ...

to allow bleeding of the braking system. Whenever service work has been performed on the braking system which might have introduced air into the system (i.e. the hydraulic system has been opened), the air must be bled out. This is necessary for correct operation of the system, because air compresses (as opposed to

brake fluid Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It wo ...

which is not compressible). Air in the system reduces the maximum pressure applied to the brake pistons and can lead to loss of braking ability.

Hydraulic operated clutches

Cars with manual

gearshift A gear stick (rarely spelled ''gearstick''), gear lever (both UK English), gearshift or shifter (both U.S. English), more formally known as a transmission lever, is a metal lever attached to the transmission of an automobile. The term ''gear sti ...

use a

clutch A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...

. Clutches are controlled from pedal by

bowden cable A Bowden cable ( ) is a type of flexible Wire rope, cable used to transmit mechanics, mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing. The housing is generally of composite construction, ...

or hydraulic systems, some use the fluid in common with the brake system. As a dead end of the hydraulic system, the slave cylinder is equipped with a bleed screw.

References