A check valve lets oil pass one way and blocks it the other, and in a hand pump or jack it is the part that holds the load between strokes. A ball check valve seals on a circle: a 10 mm ball on a 45° seat seals at 7.07 mm diameter, so at 3.75 MPa the oil presses it shut with 147 N. Test a check valve under pressure with oil, since blowing through it only proves it seals against a breath.
Part of Hydraulic Systems
A check valve is a one-way gate for fluid: a ball or a flap lifted by forward flow and pushed back onto its seat by reverse flow.
The Ball Seals on a Circle, and Its Area Sets the Closing Force
Following the carrier, make the seat as a hole slightly smaller than the ball with a 45° chamfer, and drop in a hardened steel ball of 8-12 mm with a light spring and a retaining plug. The ball touches the cone along a circle. For a cone at 45° the contact circle has a diameter equal to the ball’s diameter times sin 45°, a geometric result:
- Ball diameter 10 mm; sealing diameter = 10 x 0.7071 = 7.07 mm
- Area inside the sealing circle = π / 4 x 0.00707² = 3.93 x 10⁻⁵ m²
- Closing force from the downstream pressure at the pump maximum of 3.75 MPa (the 8:1 row of Single-Acting Pump) = 3,750,000 x 3.93 x 10⁻⁵ = 147 N
- At 20 MPa, the press pressure used in later articles, the same valve is closed by 785 N
This is why check valves seal better with pressure: pressure pushes the ball harder onto the seat. It also means the seat must withstand that force repeatedly, which is why the ball is hardened and the seat is formed into a material that gives slightly.
What You Need
- A hardened steel ball, 8-12 mm, round and smooth; a ball with a flat or a scratch will leak.
- A seat drilled slightly smaller than the ball, chamfered at 45°, in a body of mild steel or bronze. Bronze is also used for fittings per the boiler article.
- A light spring that holds the ball on its seat in the closed direction without a flow.
- A retaining plug or screw cap that can be removed to clean the valve.
Method: Make and Seat a Ball Valve
Step 1: Drill the seat hole and chamfer it to a cone with a smooth finish.
Step 2: Seat the ball. Place the ball on the chamfer and give it a few light taps with a brass punch to bed it, so the seat forms to the ball and a thin sealing ring results (a common practice, recalled and not from the corpus). Clean away all metal dust.
Step 3: Assemble with spring and plug, then bench-test.
Step 4: Test under oil pressure. Hold the valve in the closed direction at working pressure for several minutes with a gauge and watch for a pressure drop or weeping. Blowing through it with your mouth, as the carrier suggests, is a quick first check that the ball seats and the valve passes air one way only, but a valve that holds a breath can still leak at megapascals.
Flap Valves for Low Pressure
A leather flap over a hole is the simpler valve, as in the well pumps of Hand Pumps. It suits the low pressures of water, not the megapascals of oil; no pressure limit is sourced for it here, so keep it to low-pressure suction sides.
Common Mistakes
- A seat damaged by dirt. A grain of grit under the ball holds it off its seat and the valve leaks. Keep the oil filtered.
- A spring too strong. It holds the valve shut against a weak suction and starves the pump.
- Trusting one check valve to hold a load. A single seat can leak; support the load mechanically.
Safety
The check valve holds the load, until it does not
In a hand pump or jack the outlet check valve is what keeps the oil, and so the load, in place between strokes. If grit lifts the ball or the seat wears, the load sinks without warning. Never work under, or put hands in, a load held by oil alone; use a stand or blocks. A pinhole in a pressurised valve body injects oil through skin; never search for a leak with a hand, use card or wood.
What’s Next
- Learn next: Hydraulic Cylinders — what the pump and its valves feed
- Combine with: Directional Control Valves — steering oil to either side of a cylinder