Section 21 · Design & Build
Valves & the air chamber
A ram has two moving parts, and almost everything about its behaviour — tempo, hammer strength, how much it delivers, how long it lasts — is decided by how those two are shaped, seated and weighted, and by the cushion sitting above them.
The waste (clack) valve
This is the valve that does the work, and the one the operator tunes. It hangs open under its own weight while the water accelerates past it; when the flow is fast enough, drag and suction close it, and the column slams. Its weight sets how fast the water must move before that happens.
Three things you can change
- The weight — how much force it takes to close, and therefore the closure velocity, the hammer strength and the beat rate. Weight moved outward along the arm makes the valve heavier: slower, harder, weightier beats, better for high lifts. Inward gives faster, gentler beats for low lifts. This is the only dial that matters day to day; the method is in tuning the beat.
- The stroke (lift) — how far the valve travels between open and shut. Too little and the water cannot get up speed before the valve closes, so the hammer is weak. Too much and it takes too long to close, so part of the pulse escapes and the beat gets slow and soft.
- The seat — the surface the valve closes against. A resilient rubber facing (a bicycle inner tube is the traditional field repair) seals hard and quietly and is easily renewed. A metal seat lasts longer but seals less forgivingly, and grit embeds in it.
A healthy ram beats steadily, typically 30–100 times a minute. Flutter means the valve is too light; a laboured beat that stalls means it is too heavy for the flow. Both are in the troubleshooting matrix.
Two details that separate a reliable valve from a temperamental one: the valve must be guided but free — any wear in the guide turns into a valve that hangs up at one end of its stroke — and it must self-start, meaning that from rest it can be pushed down and left to catch. If it needs coaxing on every restart, the fault is usually stroke or weight, not the pump.
The delivery check valve
The check valve is the one-way gate above the pump body. When the slam's pressure spike arrives it is pushed open and admits water into the chamber; when pressure falls back below the delivery head it reseats and holds the column, so water already lifted cannot run back downhill. It is a bought part rather than a made one, and three families cover almost every ram:
| Type | How it seals | Where it suits |
|---|---|---|
| Swing check | A hinged flap swings onto a seat, driven by flow and gravity | The classic choice, and the classic DIY part; simple, cheap, serviceable. Needs to be mounted so gravity helps it close. |
| Lift / poppet check | A disc rises off a seat along a guide, and drops back | Compact, tolerant of orientation; common in commercial rams, where the disc and spring are replaceable items. |
| Spring-loaded or diaphragm check | A spring or elastomer holds the disc shut until the pressure overcomes it | Faster closing, less backflow; needs a spring light enough not to throttle the delivery. |
- Cracking pressure matters. The valve must open readily at the spike's pressure but hold firmly against the delivery head. A stiff spring wastes beats; a slack one lets water slip back.
- Orientation matters. It must be able to pass flow upward and only upward. A swing check fitted upside down, or leaning the wrong way, can fail to reseat.
- The snifter belongs on the chamber side of it. Air admitted below the check would be pushed back out through the waste valve; admitted above, it stays in the cushion where it is wanted.
The air chamber
The chamber is the shock absorber, and it does three jobs at once: it converts the destructive spike into useful delivery pressure, it keeps water moving up the delivery line between beats instead of starting and stopping, and it protects the delivery pipework from the hammer. Without it the delivery line bangs and the check valve takes the whole shock.
Its size is set by how much pressure swing you are willing to accept, and the arithmetic is derived from the polytropic relation — the calculator and the reasoning are on design tools. The practical rule most field guides quote is 20–50 × the water delivered per beat, which lands in the same place.
Four ways to build one
| Style | How it works | Trade-off |
|---|---|---|
| Plain vessel + snifter | A sealed cap of air above the water, topped up by a small hole that inhales a bubble each beat | Cheapest and most common. The air dissolves slowly into the water, so the chamber needs periodic recharging — see recharging. |
| Inner tube inside the vessel | The classic field trick: an inflated tube keeps the air in a bag, out of contact with the water | Ends the loss of air almost entirely, for the price of a bicycle tube. |
| Bladder or diaphragm vessel | A purpose-made flexible barrier separates air from water, precharged through a valve | Behaves best and needs no snifter; costs the most and the bladder is the part that ages. |
| Steel tank | A small pressure vessel with a snifter or a charge valve | Toughest option and the one to choose when the duty is large or the consequences of failure are serious. |
Charging
A chamber with a precharge valve should be set to roughly the static delivery pressure — the head from the ram up to the tank — with the pump stopped. Charge it much higher and the chamber cushions nothing; leave it empty and the first beats have to compress water, which they cannot do.
- Pressure-rated vessel only. Never thin-wall drainage pipe; the spike is tens of times the static head. See pipe & materials.
- Never stand over a beating chamber, and never pressure-test one with compressed air.
- Consider a relief valve on the delivery side. It is cheap insurance against a blocked line, and it is the difference between a leak and a rupture — see the pressure-vessel case.
Ram types compared
Every ram uses the same engine — momentum, a stopped column and a pressure wave — but the valve arrangement differs, and the differences matter when you are choosing or specifying one.
| Type | The idea | Worth knowing |
|---|---|---|
| Classic clack | A weighted flap waste valve and a swing check | The default. Simple, repairable anywhere, and what almost every DIY build and village programme uses. |
| Venturi waste valve | A concentric valve around the inlet replaces the flap | More compact and smoother-running; sold commercially. The principle is unchanged — only the geometry. |
| Compound ram | Drive water and delivered water travel in separate circuits and never mix | Lets you lift clean water using a dirty stream as the power source. The elegant answer for potability — see water quality. |
| Diaphragm-chamber ram | A clack valve with a purpose-made bladder chamber | Quieter, steadier delivery, no snifter to watch; the vessel is the maintenance item. |
Whatever the arrangement, the diagnosis is the same: listen to the beat, and judge the machine by the water arriving at the tank. Measuring that properly is monitoring & records.