TL;DR: A single-acting telescopic hoist takes its entire oil volume out of the reservoir during one lift, so the tank must be sized from the swept volume of all its stages, not from pump flow. A four-stage hoist with 140, 115, 90 and 65 mm bores over a 3,100 mm stroke moves about 26.5 litres, which calls for a roughly 40-litre baffled tank with the suction port still submerged at full extension. Undersize that tank and the pump cavitates long before the cylinder is at fault.
On a tipping trailer, the part replaced first is usually the pump and the part undersized from the drawing stage is usually the tank. A telescopic hoist is a displacement system, not a circulating one: oil leaves the reservoir, fills four nested stages, and stays there while the body is up. That makes the reservoir a storage vessel first and a cooling vessel second.
The three-to-five-times rule belongs to a different machine
For a circulating industrial power pack, tank volume is set by dwell time: oil needs long enough in the tank for air to rise out and solids to settle. Hydraulics Online's technical knowledge hub gives the figure — tank volume "should be between 3 and 5 times the pump flow/minute, with 2.5 times flow as a minimum". Sound advice where oil returns continuously.
Apply it to a tipper and it breaks both ways. A 40 L/min PTO pump at three times flow asks for 120 litres, which no trailer frame will carry. At 2.5 times a 12 L/min pump you get 30 litres, a plausible number from the wrong input.
What matters is how much oil the cylinder swallows. Everything else follows from it.
Swept volume, stage by stage
Each stage displaces pi/4 × bore² × stroke. The stages extend in series, so the volumes stack: when the fourth stage moves, the first three are already full and their oil is not coming back.
Take a four-stage front-end hoist, 3,100 mm total stroke, run at 40 L/min:
| Stage | Bore (mm) | Stroke (mm) | Swept volume (L) | Cumulative oil drawn (L) | Time at 40 L/min (s) |
|---|---|---|---|---|---|
| 1 (outer) | 140 | 700 | 10.8 | 10.8 | 16 |
| 2 | 115 | 750 | 7.8 | 18.6 | 12 |
| 3 | 90 | 800 | 5.1 | 23.7 | 8 |
| 4 (inner) | 65 | 850 | 2.8 | 26.5 | 4 |
| Total | — | 3,100 | 26.5 | 26.5 | 40 |
Read the cumulative column as a fuel gauge. When the last stage finishes, 26.5 litres have left the tank and none returns, so whatever remains is all the pump can draw.
The time column carries its own lesson. Flow is constant while area falls, so each stage runs faster and the final 850 mm takes four seconds. A hoist that slows near the top is fighting something, not obeying geometry. The force side is in our multi-stage telescopic cylinder sizing article.
Two tank numbers, not one
Usable volume is the oil between the low mark and the full mark; total volume includes the air space above the oil. Confusing the two is where trailer tanks go wrong, because a 40-litre tank with the suction port 30 mm off the bottom gives up perhaps 36 litres of usable oil.
Three limits hold on a tipper:
- Usable volume at least equal to swept volume. Below that, the pump draws air over the last part of every stroke.
- Total volume around 1.5 times swept volume. That keeps the tank about a third full with the body raised, so the suction port stays covered on a slope.
- Five to eight percent headspace. Mineral oil expands about 0.07 percent per Kelvin, so a 50 K swing adds 3.5 percent. A tank filled cold to the brim pushes oil out of the breather.
For the example above, 26.5 litres of swept volume points to a 40-litre tank. Checking takes one lift: fill to the mark with the hoist retracted, raise the body fully, read the sight glass. If the level is at the suction port, the tank is too small or the system was underfilled.
What a starved tank looks like in service
Air in the oil never announces itself as a tank problem. It arrives as four symptoms misread as pump wear.
- Whine that follows pump speed. Cavitation tracks the pump, so it sounds like a failing pump. It is the pump running short of oil.
- Foam on the dipstick or sight glass. Aerated oil holds bubbles for minutes. Clean oil settles flat within seconds of the pump stopping.
- A hesitation at the start of each stage. The pump re-primes after drawing air at the end of the previous stroke.
- Oil too hot to hold after three cycles. Air compression adds heat that a small tank cannot shed between lifts.
The field discussions agree. On a TractorByNet thread about a dump trailer that would not lift a load, one member put the arithmetic bluntly: "A true single acting cylinder needs a pretty big reservoir. Bigger than most little 12v power units have." The same thread calls this "the single biggest reason most dump trailers are power down" — a power-down circuit only makes up the rod volume, not the whole cylinder. An older thread on the same forum describes a tank running dry part-way up and overflowing on the way down.
The tank is a component, not a box
Volume alone does not fix aeration. A reservoir needs a baffle between the return and the suction port, so return oil has to travel before it can be drawn again. The return must terminate below oil level, because a return that splashes into the air whips in air faster than any baffle can release it. A filtered breather and a suction strainer sized for the pump inlet are part of the sizing too. A tank welded from 3 mm sheet without a baffle drums under a 40 L/min return and cracks at the bracket.
Five checks before the pump comes off
- Oil level at full extension. The only level check that means anything here.
- Foam and colour. Milky oil has water in it, persistent foam has air in it. Neither is a pump fault.
- Temperature after three loaded cycles. Mobile hydraulics sit around 50 to 60 °C. Much above that points at the reservoir or the relief.
- Suction line condition. A hose that collapses under suction, or a strainer fouled with swarf, starves a healthy pump.
- Relief setting against stage force. If the tip stops at two-thirds height with the relief open, the smallest stage wants more pressure than the valve allows.
What we build
HCIC has made telescopic and welded cylinders since 1998, across three plants and one R&D centre with 20+ engineers among 250+ staff. Output runs at 210,000 cylinders a year, 95 percent exported, to ISO 6020, ISO 6022 and ISO 10100, with a 3000 PSI proof test on the telescopic range before dispatch. Every cylinder ships with an 18-month warranty and a full maintenance kit.
We quote from a drawing or a worn sample within 48 hours, minimum order one piece. If the tank is the weak point on an existing trailer, our 12V hydraulic power units pair a reservoir with the pump and valve for the same displacement, and the agricultural valve and manifold range covers load-sensing circuits. Cylinders for this duty are listed under transportation hydraulic cylinders, with bore and stroke combinations in the telescopic cylinder model catalog. Send the stage bores, strokes and pump flow and we will return the swept volume with the quote.
Frequently asked questions
How much hydraulic oil does a dump trailer telescopic cylinder hold?
Add the swept volume of every stage: pi/4 × bore² × stroke, from the largest stage to the smallest. A four-stage hoist with 140, 115, 90 and 65 mm bores over a 3,100 mm stroke holds about 26.5 litres at full extension. Measure the stages on the drawing, not from the tonnage rating.
What size hydraulic tank does a telescopic tipper need?
Usable tank volume should be at least the swept volume of the cylinder, and total volume about 1.5 times it. For the 26.5 litre example that means roughly 40 litres total, so the tank is a third full with the body up. The 3 to 5 times pump flow rule used on circulating systems is not the right basis.
Why does my dump body lift fast and then slow down near the top?
Stage area falls as the hoist extends, so speed should increase, not decrease. A lift that slows near the top is usually starved of oil, has a blocked strainer, or has hit a relief setting the smallest stage cannot work against. Check the oil level with the body raised before touching the pump.
Can I just add oil to fix a slow telescopic hoist?
Only if the level is low at full extension. If the tank still shows oil with the body up, adding more overfills it, and the returning oil pushes it out of the breather. Find where air enters the suction line instead.