Hydraulic systems in interlocking brick-making machines: components, pressure control, and maintenance

The hydraulic power unit, oil cooler, and main cylinder of an interlocking brick press. Image: AI-generated for this article. It shows no real machine or brand. Most powered interlocking brick machines form each brick with a hydraulic press. A soil-cement mix fills a steel mold, and a cylinder drives a press head into it. The hydraulic system sets press force, cycle time, and much of the downtime. This article explains how it works, how pressure relates to brick quality, and how to keep it reliable.

How does the hydraulic press cycle work?

1. Fill. A feeder box slides over the mold and drops a charge of mix.

2. Press. A directional control valve sends pump flow to the cap end of the main cylinder. The press head compacts the mix.

3. Hold. The press holds force for a short dwell.

4. Eject. The valve shifts. The cylinder lifts the mold or pushes the brick out.

5. Return. The flow goes to the rod end, and the press head retracts.

Manual lever presses have no hydraulics and are outside this article.

What are the main hydraulic components?

The relief valve limits the power the system can generate. That protects components and workers.

How does hydraulic pressure relate to brick quality?

Three pressures are often confused:

• System pressure is the gauge reading at the HPU. The relief valve caps it.

• Cylinder force is that pressure multiplied by the piston area.

• Compaction stress is the cylinder force divided by the total brick face area in the mold.

The ratio of piston area to mold area decides how much gauge pressure reaches the brick.

Worked example (illustrative numbers, not a datasheet):

Four bricks per quarter stroke the stress. Mould friction lowers it further. A machine listed at 30 MPa therefore does not put 30 MPa on the brick. Ask for bore, bricks per stroke, and press force.

For scale, the Auroville Earth Institute gives about 2 to 4 MPa for manual presses. Published research describes 25 MPa as comparable to the most powerful compressed earth brick presses on the market.

Higher pressure alone does not guarantee a better brick. Research on compressed earth blocks shows that:

• Density rises with compaction pressure, but less than linearly.

• The optimum moisture content falls as compaction effort rises. Mix water that suits the press.

• Soil quality, stabilizer type and amount, workmanship, and curing all affect strength.

• Higher pressure cannot make up for a poorly graded mix. Water durability fell as compaction pressure rose in one study.

• Compaction time matters less. Strength grew with longer consolidation, but raising pressure was more effective.

How do heat, filtration, and cooling affect the circuit?

A system's heat load equals the power it loses. Heat appears wherever oil drops pressure without doing work. Holding pressure over the relief valve during each dwell heats the oil quickly.

Hot oil thins and leaks past seals, so the press slows. Above 82°C, most seal compounds are damaged and oil degrades faster. Viscosity can become too low well below that. One widely used estimate halves oil life for every 10 °C above 60 °C. Installed cooling capacity typically equals 25 to 40 percent of input power.

Brickyards are dusty. Filter makers attribute the majority of hydraulic failures to solid contamination. Contaminated oil is also a main cause of seal leakage. In practice:

• Fit a return-line filter with a clogging indicator.

• Fit a filtered breather on the reservoir.

• Keep cooler fins clear of dust.

• Track cleanliness with ISO 4406 particle counts.

What are the common hydraulic faults?

Preventive maintenance and safe work

• Daily: check oil level, oil temperature, leaks, and guards.

• Weekly: clean cooler fins and read the filter indicator.

• At the maker's interval: change filter elements, sample the oil, and verify the relief setting.

Seal life suffers from high temperature, pressure peaks, aggressive fluid additives, and side loads. Rod surface finish also matters.

ISO 4413 sets general safety rules for hydraulic systems on machinery. On a brick press, apply them as follows:

• Hoses: specify hose to SAE J517 or ISO 18752. Rate it above the relief setting. Operating pressure is generally one quarter of minimum burst pressure, and pressure surges can exceed relief settings.

• Leak inspection: Never feel for leaks by hand. A pinhole jet can pierce the skin. Use cardboard.

• Guarding: guard the press head and mold zone. Never reach in while the HPU runs.

• Depressurization: stop the motor and lock out power. Relieve all stored energy before work, as OSHA 29 CFR 1910.147 requires. Bleed any accumulator, which can release its energy within seconds. Confirm zero on the gauge.

Reliability

Seals, hoses, filter elements, and mold wear plates are consumables. Keep spares on site. Log oil temperature and cycle time. A rising trend in either is an early warning.

Key hydraulic specifications to check

• Rated system pressure and relief setting

• Cylinder bore, rod diameter, and stroke

• Press force, bricks per stroke, compaction stress

• Pump type, flow, and motor power

• Reservoir volume and cooler capacity

• Filter positions and ratings

• Hose standard and working pressure

• Cycle and dwell time

Applying the checklist to a datasheet

Published datasheets often give only part of this list. For example, Lontto's automatic interlocking brick-making machine, the LT7-10, is listed at a hydraulic pressure of 30 to 60 MPa, seven bricks per cycle, a 5 to 10 s moulding period, and 22 kW. The cylinder bore and press force are not published. Without them, the compaction stress on each brick cannot be worked out. Ask any supplier for those figures, a hydraulic circuit diagram, and the relief setting before comparing machines on pressure.

Disclosure: Lontto contributed to this article. Specifications are as Lontto states them and were not independently tested.