Frictional resistance

Frictional resistance shapes nearly every work step in concrete demolition, specialist deconstruction, and rock processing. It determines whether grapples hold securely, wedges expand in a controlled manner, cutters separate cleanly, and hydraulic systems transmit force efficiently. For the products and application areas of Darda GmbH – from concrete demolition shears and hydraulic rock and concrete splitters to combination shears, multi cutters, steel shears, tank cutters, and the associated hydraulic power packs – understanding friction is a practical foundation to ensure performance, precision, and occupational safety in use. In addition, frictional resistance governs energy flow and tool wear across the entire process chain, making it a decisive parameter for consistent quality and predictable cycle times.

Definition: What is meant by frictional resistance?

Frictional resistance is the tangential counterforce that acts on contact surfaces and inhibits or prevents relative motion. It arises from the micro-interlocking of rough surfaces and from adhesive forces. A distinction is made between static friction (no sliding, maximum holding force) and kinetic friction (motion with resistance). The friction force FR is approximately proportional to the normal force FN and is described by the coefficient of friction µ (FR = µ · FN). In practice, µ is influenced by material pairing, surface condition, contact pressure, moisture, temperature, and contamination. Under dry conditions, the friction force is largely independent of the apparent contact area and is better described by µ_s (static) and µ_k (kinetic), both dimensionless and condition-dependent.

Physical fundamentals and coefficients of friction

Friction arises at real contact asperities. The higher the normal force and the more pronounced the surface roughness, the stronger the interlocking and thus the frictional resistance. Static friction is usually greater than kinetic friction; when transitioning from sticking to sliding, the friction force drops. For steel on mineral materials (concrete, natural stone), µ spans a wide range depending on moisture, roughness, and contact pressure. Dry, rough contacts show higher friction values than smooth, wet, or dusty surfaces. These relationships directly determine how securely shears grip, how effectively splitting wedges anchor, and how high the energy losses are in joints and hydraulic systems. Indicatively, steel on dry, rough concrete can reach µ ≈ 0.6 to 0.8, on wet concrete µ ≈ 0.3 to 0.5, and on dusty or slurry-coated surfaces µ can fall to ≈ 0.2 to 0.4, with local pressure and sliding speed further modulating these values.

Frictional resistance in demolition and splitting processes

In concrete demolition and specialist deconstruction, friction governs holding force, process stability, and efficiency. With concrete demolition shears, friction supports the gripping and crushing action; with stone and concrete splitters, friction enables the reliable bracing of counter-wedges in the borehole. In steel and tank dismantling, friction influences chip formation and edge wear. At the same time, friction in hydraulic power packs appears as a loss factor expressed through viscosity, flow, and seal friction. Superimposed vibrations and micro-motions can cause fretting and accelerate wear at contact points, which underscores the value of clean surfaces and stable kinematics.

Concrete demolition shears: gripping, crushing, shearing

Concrete demolition shears transmit high normal forces into the concrete via profiled jaw faces. The frictional resistance at the contact surfaces prevents slipping, stabilizes the component, and guides fracture lines. Sufficiently high static friction is advantageous because it converts gripping force into shear and tensile stresses within the concrete. At the same time, kinetic friction generates heat and wear on teeth and joints, which requires maintenance and controlled working methods. Optimized jaw alignment and short, precise work strokes reduce unnecessary sliding and help avoid surface glazing of the teeth.

• High friction at the jaw faces promotes controlled gripping and breaking.
• Dust, mortar residues, or moisture can lower the coefficient of friction and lead to slip.
• Contact pressure, tooth geometry, and component edges influence local friction force distribution.
• Low friction in joints increases overall efficiency and the controllability of the shear.
• Allow cooling intervals during intensive cycles to limit heat-induced wear at contact points.

Stone and concrete splitters: wedge, abutment, and borehole friction

Hydraulic split cylinders work on the wedge and counter-wedge principle. The counter-wedges brace themselves against the borehole wall through friction and form-fit while the wedge builds the splitting force. Decisive factors are borehole quality, diameter fit, cleanliness, and moisture content. Dry, clean boreholes with suitable roughness provide high static friction and reduce the risk of wedges “creeping” along. Too little friction – for example due to a water film, slurry, or smoothness – can reduce effective splitting force and delay crack initiation. Appropriate wedge angles and correctly matched spacers further stabilize the bracing without overloading the borehole wall.

Combination shears, multi cutters, steel shears, and tank cutters

With cutting tools, friction acts in three locations: at the cutting wedge (workpiece–bevel contact), on sliding surfaces of the jaws, and at bearing points. Adequate frictional resistance at the component support stabilizes the workpiece, while reduced friction at joints and sliding surfaces lowers the required force and reduces wear. In tank and vessel dismantling, controlled friction supports a cool, low-spark separating effect when the cutting method relies on shearing instead of thermal cutting. Cutting speed, clearance, and bevel condition influence heat input and chip evacuation, and thus the balance between reliable hold and minimal tool wear.

Influencing factors in practice

Friction is not a fixed value but the result of many boundary conditions. Those who actively shape these factors control holding force, process reliability, and energy efficiency.

• Material and structure: concrete compressive strength, aggregate hardness, reinforcement ratio; rock type, bedding, and jointing.
• Surface condition: roughness, edges, microcracks; smoothing due to wear.
• Medium: dry versus moist/slurry; dust and particulates as a separating layer.
• Contact pressure: local pressure increases static friction; excessive peaks promote crumbling and tear-off.
• Temperature: heating often reduces viscosities (hydraulics) but can change friction values and wear behavior at contacts.
• Kinematics: angles, wedge angles, feed rate, and relative motion influence the sliding proportion.
• Hydraulic condition: oil quality, filtration, pressure losses in lines and valves; seal friction.
• Alignment and support geometry: seating on edges or ribs shapes local stress and friction distribution.
• Environmental influences: frost, de-icing salts, and fine sludge can drastically alter µ and accelerate wear.

Measurement, estimation, and practical reference values

In practice, friction is rarely measured directly. Simple holding and slip tests are common, from which permissible gripping loads or splitting conditions can be derived. Typical friction values for steel on concrete or natural stone vary significantly with roughness and moisture. Dry, rough contacts generally provide higher values than wet or dusty surfaces. For on-site decisions, a conservative estimate is sensible and should be combined with safety margins. As a quick method, inclined-plane or pull-off tests with known normal forces provide indicative µ values that reflect local conditions.

Simple field checks

1. Clean and dry the contact surfaces.
2. Apply with low force and watch for slip.
3. Increase contact pressure step by step, observe the component’s reaction.
4. Optimize gripping position and angle, then apply full load.
5. Document surface state, moisture, and tool setup to track comparability.
6. Repeat checks when weather, temperature, or material changes are observed.

Optimization: intentionally increase or decrease friction

Depending on the task, more or less friction is useful. During gripping and bracing, friction increases process stability; in joints and flows, lower friction reduces losses and heat. Targeted surface preparation and condition monitoring prevent unexpected transitions from static to kinetic friction.

Increase friction at contact surfaces

• Clean contact zones, remove loose particles, minimize moisture.
• Increase contact pressure through correct tool positioning (e.g., on edges or in load-bearing zones).
• Keep profiled jaw faces intact; refurbish or replace worn teeth in time.
• Select a suitable wedge/borehole pairing; produce boreholes to size and rough, but not polished.
• Do not introduce lubricants on gripping or bracing surfaces.
• Use serrated or textured contact inserts where design permits to raise µ without excessive pressure peaks.

Reduce friction in drives and joints

• Lubricate joints and bearings according to manufacturer specifications; avoid contamination.
• Select hydraulic oil with suitable viscosity; keep oil temperature within the target range.
• Check filter condition and tightness to minimize pressure losses and internal friction.
• Route hoses with large bend radii, avoid kinks.
• Match seal material and surface finish to pressure and speed to limit stick-slip and leakage.

Safety and work practices

Frictional resistance affects control over the component. Unexpected slip can abruptly shift parts. Safe working includes stable supports, braced components, controlled force ramp-up, and clear team signaling. In splitting work, crack direction and relief paths should be planned; with shears and cutters, danger zones for fragments and falling parts must be avoided. A good view of the contact zones helps detect sliding movements early. Test holds at partial load and defined exclusion zones around the work area provide additional safety in borderline friction conditions.

Examples from application areas

In concrete demolition and special deconstruction, a clean contact surface for concrete demolition shears improves holding force and reduces uncontrolled fracture break-offs. During strip-out and cutting, low friction in joints reduces operating effort, while high friction at the component support avoids slip. In rock excavation and tunnel construction, dry boreholes secure the frictional bracing of counter-wedges in stone and concrete splitters; moist joints require conservative load assumptions. In natural stone extraction, friction along existing joints favors directed crack propagation. In special operations – for example with contaminated surfaces or coated components – cleaning and verification measures prior to force application are especially important. Across all cases, consistent documentation of surface condition and moisture content supports reproducible setup and safer force application.

Design and maintenance: deliberately account for frictional resistance

Darda GmbH’s products use friction where it provides holding force and process reliability, and minimize it where it costs efficiency. Design, material selection, and surface finishing support this goal. In practice, inspection and care secure the intended friction properties over the entire service life. Torque levels on bolted joints and jaw attachments influence contact pressure distribution and, with it, effective friction and wear progression.

• Inspect the jaw faces of the concrete demolition shears; do not allow edges and teeth to become “polished smooth.”
• Produce boreholes for stone and concrete splitters to size, remove flushing and slurry residues.
• Lubricate joints and bearings, check play, inspect seals.
• Check hydraulic power packs for oil condition, filters, and operating temperature; avoid pressure losses.
• Keep contact surfaces free of oil, grease, and silicone-containing agents.
• Verify torque on fasteners critical to jaw guidance and bearing preload to maintain consistent kinematics.

Energy, wear, and efficiency

Frictional resistance converts drive energy into heat and abrasion. In hydraulic systems, it appears as pressure loss; at contact surfaces, as heat generation and material removal. A deliberate balance – high friction for holding, low friction for movements – increases efficiency and extends the service life of tools and components. Monitoring temperatures at bearings and hydraulic return lines, along with routine visual checks of wear patterns on contact faces, provides early indicators for optimization.