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Written by G.L. Huyett Marketing Department on 06/10/2020 with 0 comments

Pressure Control Fittings

Pressure control grease fittings – also known as pressure relief Zerk grease fittings – prevent overpressurization damage to gearboxes and bearings by ensuring that pressure never exceeds a predetermined maximum.

Gearboxes and bearings function best when their internal pressure is within a range determined by their design and construction. During operation, gasses and lubricants expand as temperatures rise, potentially increasing pressure beyond the desired range.

Excessive pressure may also be introduced during regreasing: grease guns operate at significantly higher pressures than most bearings, risking overpressurization.

If the pressure exceeds a predetermined threshold, parts may wear more quickly, high‑pressure churning may reduce efficiency, seals may break, and the life of the component may be reduced. Grease control pressure fittings extend the life of machine components while facilitating fast regreasing.

What are Pressure Control Grease Fittings?

Pressure control grease fittings are safety valves or vents that maintain internal pressures below a predefined limit. If pressure increases beyond the threshold, the pressure relief valve releases gasses until the desired operating pressure is reestablished.

There are three groups of pressure control grease fittings: vents, breathers, and relief vents.

  • Vents are one‑way valves that allow air to flow in only one direction. The valve opens to release pressure and then closes to prevent airflow in the opposite direction.
  • Breathers do not restrict airflow and allow the gearbox to "breathe" in and out.
  • Relief vents have a small interior valve that relieves pressure at prescribed pressure ratings.

What are the Three Groups of Pressure Control Grease Fittings?


Vents open and close within a predetermined pressure range: As internal pressures rise, a flat seal in the vent (typically composed of rubber) opens, allowing pressure to vent. As the pressure decreases, the seal closes. The rubber seal has the added advantage of sealing the port opening while closed, thus preventing dirt, water, and other contaminants from entering.

Vent Fittings

The vent opens and closes at a predetermined pressure. The vent seals so that air is only allowed to move out of the assembly.


Breather pressure relief fittings feature specially designed ports that allow air to move in and out, creating the effect of "breathing." They have a special cap which reduces contamination. For additional contamination protection, some breathers are equipped with a felt filter.

Breather Fittings

Breathers do not restrict airflow, keeping the internal pressure at a safe operating level.

Relief Vents

Pressure relief vents open and close within a predetermined pressure range. As internal pressure rises, the plunger or check ball opens, allowing pressure to vent. As pressures decrease to below the minimum threshold, the valve closes. Relief valves are available in both a top vent and side vent design.

Relief Vents

Closed: The basic design consists of a pre‑engineered spring that controls a pop‑up valve. At peak performance, the valve is closed, sealing the bearing.
Open: Once internal pressures reach the predetermined opening psi, the pop‑up valve engages and the fitting allows pressure to escape.

What is over‑lubrication?

Gearboxes require lubrication to function, but too much grease can damage components, reduce efficiency, and collapse seals as pressure rises during operation. The temperature and pressure inside a gearbox are not constant. As components turn inside gearboxes, the friction of the moving parts produces heat, causing the interior liquids and gases to expand and internal pressure to increase. If not controlled, the excess pressure can damage gearbox components.

How to Prevent Mechanical Damage From Over‑Lubrication:

Relief valves prevent component damage from over-lubrication by venting excess pressure. As you can see in the image below, pressure in a gearbox without a valve increases as temperatures rise until seals around the shaft break, allowing the lubricant to leak.

The gearbox with the relief valve fitted successfully vents excess pressure before it is damaged. This pressure relief allows the gearbox to operate safely at a variety of RPM's and various atmospheric conditions.

Left: In a rest condition, fluid and gases inside the gearbox are ambient.
Middle: While in operation, the temperature inside the gearbox increases causing fluid levels and pressures to rise.
Right: If not monitored, the increased pressure can blow out seals.

Over Lubrication and Prevention

How to Install Pressure Relief Vent Grease Fittings

Pressure control fittings are typically installed above the stationary fluid line so that pressure is reduced by the venting of gasses. If they are fitted below the fluid line, grease may be vented instead.

Pressure relief vent grease fittings are available in a wide range of pressure ratings to suit many environments. Be sure to have the proper pressure control fitting with the correct pressure rating for the application. If a fitting with the wrong pressure rating is installed, venting may not occur before efficiency is affected and components are damaged.


Can you adjust a pressure relief valve?

Yes. We stock a range of pressure relief valves that can be adjusted within a PSI range. We also stock valves rated for a predetermined pressure.

Grease fitting pressure relief valve maintenance

Relief valves should be regularly inspected for damage, blockages, and wear. If they are damaged, they should be replaced immediately. Inspection and maintenance tasks include:

  • Checking for dirt, dust, and debris: Some pressure relief valves integrate a cap to reduce the ingress of debris, but it is possible for the opening on any valve to become blocked over time, preventing the valve or vent from functioning correctly.
  • Wear and tear: Relief valves are mechanical components that wear over time.
  • Correct valve placement: Valves should be placed above the lubrication fluid level of a component to ensure that gasses are vented.

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