Head pressure is the force the grinder's own weight presses onto the grinding shoes per unit of contact area, and it controls cut rate, block wear and finish even before grit or bond are chosen. On walk-behind machines from 30 to 200 kg, pressure bands run roughly 10 to 35 kPa, or 0.10 to 0.35 bar; too little pressure leaves scratches and haze, too much overloads the motor and rounds the block face.

This guide serves factories, distributors and contractor procurement teams balancing grinder weight against head pressure for concrete grinding and polishing. It covers pressure bands by stage, counterweight methods and their side effects, matching shoe count to motor power, compensation on uneven floors, and the point where a heavier machine is the right purchase rather than added weight.

How Machine Weight Becomes Head Pressure

Weight acts through the head onto the shoes; pressure equals contact force divided by total shoe area. A heavy machine on wide-area shoes can read light in pressure, while a light machine on few narrow shoes reads hard. Pressure rises with added mass and drops when contact area grows, so both weight and shoe layout must be set before checking cut.

Read two figures on site: machine and head weight in kg, and total shoe contact area from shoe size and count. Common heads carry 6 to 8 shoes, each about 100 by 100 mm, giving 0.06 to 0.08 m² of contact. Hand-held grinders run 2 to 5 kg and rely on operator feed.

Pressure Bands by Grinding Stage

The table pairs metal-bond and resin stages with a target pressure band, shoe count and speed. Metal 16, 20, 30 and 40 open the profile; resin 100, 200, 400, 800, 1500 and 3000 refine the finish.

Pressure follows the abrasive load: coarse passes carry the most force, fine passes carry the least. Raising pressure on a fine pad burns the resin face; dropping pressure on a coarse block leaves the profile untouched.

Adding Counterweight: Methods and Side Effects

When the stock machine undercuts a hard base, weight can be added in three ways: bolt-on plates over the head, removable bar weights on the side rails, or a weighted press block above the shoes. Each adds 10 to 25 kg per side and shifts the center of gravity down.

Side effects appear within 20 to 40 m of grinding. Extra mass raises bounce on joints and high spots, wears shoes unevenly along one edge, pulls current 15 to 20 percent above nameplate, and strains frame and drive joints. Overweighting also flattens the float, so a small height change transfers straight to one shoe.

Block Count, Contact Area and Motor Power

Shoe count sets how the weight spreads; motor power sets how fast the face can be driven. A 5.5 to 7.5 kW head pairs with 6 to 8 shoes in normal concrete; an 11 to 15 kW head can carry 8 to 12 shoes on hard abrasive bases. Too many shoes starves each cut; too few loads each shoe and deepens wear.

Match current to load after setup. Sustained draw above 15 to 20 percent over nameplate means the head pushes too hard or the shoes are dull; a weak, pulsing draw means pressure cannot reach the base. Keep water flow 3 to 8 L/min so slurry does not build up.

Floor Level Variation and Pressure Compensation

Few slabs sit flat: PTV runs 3 to 5 mm over a 2 m straightedge, with slopes, trowel ridges and patch repairs. On high spots the shoes bite harder and pressure spikes; in low spots they ride light and leave haze. A rigid head follows the slab; a floating head compensates by shifting weight across the shoe set.

Check level before grinding: mark high spots under 300 to 500 lux oblique light, and watch current as the head crosses them. On a slope, move slowly and keep the head level; let the shoe set, not the frame, carry the contact. Recheck pressure after a 20 to 30 m trial pass.

When to Change Machine Instead of Adding Weight

Added weight is a short fix; these signals say the machine class is undersized and the purchase should be a heavier or wider head.

• Current stays 20 to 30 percent over nameplate even with sharp shoes and clean water.
• Shoes wear unevenly on every job and the head bounces over joints or high spots.
• Required pressure sits above 35 kPa for coarse grinding, or the contact area cannot spread the load across the slab.
• Frame, drive joint or handle shows repeated strain, and added plates shift the center of gravity past the balance point.
• Daily area falls 25 to 40 percent below plan on a base the current machine cannot cut at rated speed.
• The job needs wider coverage than the shoe ring, and side weights only compensate a contact gap the machine cannot close.

Pressure Measurement and Wear Checks

Pressure is read indirectly: weigh head and machine, divide force by shoe area, and compare against the stage band. Track shoe height per pass and current draw; a cut drop after 20 to 30 m points to dull grit, not missing weight. Check shoe backs for flat seating and firm hooks or clips.

Rinse slurry after each pass and let shoes dry before stacking; trapped paste reads as glazing in the next setup. Store the machine in a dry rack within the 75 to 80% RH band, away from damp cycles.

Final Balance Checklist

Before approving a machine or weight kit, line up five facts: base hardness and aggregate, planned grinding stages, shoe size and count, motor power in kW, and daily area target. Light hand work stays on 2 to 5 kg tools; medium floor work pairs a 30 to 80 kg head with 6 to 8 shoes; heavy production moves to a 100 to 200 kg head.

Confirm pressure sits in the stage band, current stays within 15 to 20 percent of nameplate, the shoe set floats over level changes, and reorder records match the first setup. A balanced head gives even wear and predictable area per block.