Machine screw

Machine screws are central fastening elements in mechanical and plant engineering. In the context of concrete demolition and deconstruction technology, they secure load-bearing assemblies, tool carriers, and housings – for example on concrete demolition shears, hydraulic splitters, hydraulic power units, and concrete pulverizer tools by Darda GmbH. Their correct selection, installation, and maintenance directly influence operational safety, particularly under shock, vibration, and alternating loads as encountered during concrete demolition, building gutting, or rock breakout. High availability of defined spare fasteners and consistent specifications supports field service and reduces unplanned downtime.

Definition: What is meant by a machine screw?

A machine screw is a metallic screw with a metric ISO thread (coarse or fine) that connects a component via a nut or a threaded hole by force-fit and/or form-fit. When tightened, it generates a defined preload force that clamps parts over an area and transfers loads predominantly by friction. In contrast to wood or sheet-metal screws, machine screws are designed for metallic counterparts and follow standardized dimensions, head and drive types (e.g., hexagon, hex socket, countersunk head). Typical strength classes are 8.8, 10.9, and 12.9 for unalloyed/alloyed steels. In many standards, screws used with a nut are referred to as bolts; in practice, the terms are used depending on context.

Design, types, and threads of machine screws

Machine screws consist of a head, a shank with thread, and – depending on the standard – a shank portion without thread. Head and drive type determine the bearing surface, the force introduction into the component, and accessibility during assembly.

Typical head and drive types

• Hexagon head bolt (e.g., per ISO 4014/4017) for robust, well-accessible bolted joints on flanges, bearing blocks, and tool carriers.
• Cylindrical head screw with hex socket (e.g., per ISO 4762) for compact designs, for example on housings and clamping shells on concrete demolition shears.
• Countersunk screw (e.g., per ISO 10642) for a flush surface where installation space or edge clearance is required.
• Button head cap screw (e.g., ISO 7380) where a low-profile head is advantageous and only moderate preload is required.

Thread types and fit

• Coarse thread (M… x standard pitch) as the universal standard for machine joints.
• Fine thread for higher self-locking, more sensitive preload adjustment, and better utilization in thin-walled or highly loaded joints.
• Left-hand threads are rarely used, e.g., where rotation tends to loosen the joint.
• Typical tolerance classes: external threads 6g and internal threads 6H for reliable assembly and serviceability.

Length selection and engagement depth

• The thread engagement depth in steel should generally be at least 1× the nominal diameter (greater in aluminum) so that the nut or tapped hole can fully utilize the screw material.
• The smooth shank portion can be used for shear guidance (fit bolt/fit hole) when transverse forces dominate.
• Avoid having threads in the shear plane of highly loaded lap joints; target a clamp length that is several diameters long to reduce preload scatter.

Standards, strength classes, and materials

Strength and dimensional standards are decisive for machine screws. In demolition and deconstruction applications, high-strength steels with strength classes 8.8, 10.9, and 12.9 (mechanical properties per ISO 898-1) predominate. Stainless steel (e.g., A2/A4) is used less frequently in heavy machinery because the strengths are usually lower and the risk of fretting in dynamically clamped joints is higher.

Head markings identify property class and manufacturer; the material and heat treatment must match the specified class. At elevated temperatures, verify any reductions in strength and the suitability of coatings and lubricants.

• Quenched and tempered alloy steels are common for 10.9/12.9 where high preload and fatigue resistance are required.
• Non-ferrous solutions (e.g., copper alloys) are reserved for special cases such as spark reduction or specific corrosion scenarios due to lower strength.
• For mixed materials, consider insulating layers or sleeves to avoid galvanic couples under moisture exposure.

Surfaces and corrosion protection

• Zinc-flake or zinc-nickel coatings for improved corrosion protection with low friction value scatter.
• Phosphated/blackened surfaces in combination with assembly pastes for friction value stabilization.
• Hot-dip galvanizing only with suitable tolerances and required coating thicknesses – pay attention to friction coefficient and thread clearance.
• Avoiding hydrogen embrittlement in high-strength screws: select suitable coating processes and heat treatment.
• When electroplating high-strength parts, apply a controlled post-bake to mitigate hydrogen embrittlement as specified by the process.
• Use consistent lubricants and avoid unintended mixing that could alter the friction coefficient and torque-preload relationship.

Loads and design in demolition and deconstruction environments

In concrete demolition shears, hydraulic splitters, and shears, combined stresses from tension, shear, bending, and transverse loads occur against the background of impacts, shocks, and vibrations. Designs should be executed as a clamped joint with sufficient preload so that transverse forces are transmitted by friction and the screw is stressed as little as possible in shear. Relevant design parameters include preload, embedment/settlement, friction coefficient, temperature, and dynamic load spectra. In mechanical engineering, calculations are often performed according to accepted rules of engineering practice (e.g., design fundamentals analogous to VDI methodology), with appropriate consideration of component and friction value scatter.

• Separate centering/locating (pins, fitted sleeves) from clamping tasks to minimize shear on the screw shank.
• Use hardened washers under high-strength heads/nuts to stabilize friction and reduce surface indentation.
• Prefer longer clamp lengths over short stacks of hard parts to achieve a more elastic joint and better preload retention under vibration.

Selection for concrete demolition shears and stone and concrete splitting devices

In tool carriers, blade holders, flanges of hydraulic cylinders, and housing fastenings – in rock splitters and similar devices – screws should be selected so that the required preload is achieved with adequate safety and maintenance/inspection remains practical.

Practice-oriented selection criteria

• Strength class: 10.9/12.9 for highly loaded, dynamic joints; 8.8 for standard attachments and covers.
• Head/drive type: Hexagon for robust accessibility on the tool; hex socket where space is limited.
• Thread: Coarse thread for universal serviceability; fine thread for short clamp lengths or increased loosening resistance.
• Coating: Choose friction-stable systems; provide enhanced corrosion protection for outdoor installation.
• Screw locking: Chemical lockers (medium/high strength) or wedge-locking elements when strong vibration occurs.
• Washers: High-strength washers under high-strength screws; for countersunk heads ensure sufficient bearing and underhead clearance.
• Service concept: Favor standardized sizes and property classes to simplify stocking and field replacement.
• Tool access: Ensure clearance for torque tools and angle measurement as well as for visual inspection.

Installation: torque, preload, and locking

Assembly quality determines the service life and safety of a joint. The goal is a reproducible preload with minimal scatter.

1. Clean component surfaces, remove burrs and paint in the bearing area; use suitable washers.
2. Inspect screws and nuts (marking, thread condition). Document surface/lubrication condition.
3. Verify calibration status of torque/angle tools and define the tightening method (torque, torque-angle, yield control) before assembly.
4. Define the friction condition (dry, oiled, paste-lubricated). The tightening torque value depends on friction.
5. Pre-assemble in a crisscross pattern up to seating, then final tightening with a torque wrench or torque-angle method.
6. For safety-relevant joints: a second person for cross-check; mark the screw head position for visual inspection.
7. Locking means according to the application: medium-strength threadlocker for serviceable joints; mechanical locks for strong vibration.

Friction and torque

Too low a torque leads to insufficient preload and loosening; too high a torque leads to overstretching or thread damage. Calibrated tools and consistent lubrication reduce scatter. For repeated assembly, screws in highly stressed areas should be renewed. Where feasible, the torque-angle method reduces sensitivity to friction variations and increases preload repeatability.

Maintenance and inspection in tough construction site operation

In the application areas of concrete demolition, building gutting, tunnel construction, and natural stone extraction, regular visual and functional checks are advisable. Maintenance plans of Darda GmbH must be observed without exceeding the manufacturer’s specifications. Where operating data is available, condition-based intervals can supplement fixed schedules.

Checkpoints

• Retightening after the initial operating phase (settlement effects); thereafter at defined intervals according to the load profile.
• Check for rust, head rounding, thread pull-out, elongation, cracks at the head-shank transition.
• Inspect bearing surfaces for indentations/edge pressure; retrofit high-strength washers if necessary.
• Document torque values and replacement dates for traceability.
• Use paint marks on heads/nuts to quickly detect rotation or loosening during routine inspections.

Corrosion and environmental influences

Cement slurry, moisture, chlorides (e.g., in de-icing salts), and abrasive dust promote corrosion. A coordinated surface protection, suitable sealing washers at exposed locations, and regular cleaning increase durability. Avoid incompatible material pairings to prevent galvanic corrosion. At elevated temperatures (near hydraulics), verify coating and material with respect to temperature service limits.

• Fit protective caps or boots where feasible to shield exposed threads and heads from splash water and debris.
• After chloride exposure, rinse and neutralize surfaces promptly to limit pitting and crevice corrosion.

Typical failure patterns and causes

• Loosening/vibration: insufficient preload, overly smooth bearing surfaces without locking, varying friction conditions.
• Shear fracture: transverse load transfer via the screw shank instead of via friction; missing fit/locating pins.
• Thread pull-out: insufficient engagement depth or soft mating material without adequate thread length.
• Head cracks/overstretching: exceeding the permissible tightening torque; wrong strength class.
• Corrosion damage: unsuitable coating; missing cleaning; galvanic corrosion.
• Hydrogen embrittlement: high-strength screws with critical coating or missing post-treatment.
• Low-temperature brittleness: inadequate toughness in cold environments combined with impact loads.

Documentation and notes

For safety-relevant joints on devices from Darda GmbH, traceable documentation of component condition, screw specification, assembly condition (friction, torque/angle), and maintenance is advisable. Standard-compliant selection and assembly according to accepted engineering practice support operational safety. Legal requirements may vary by country of use; the notes are general in nature and not case-specific.

• Record batch/lot identification of fasteners and any applied lubricants or threadlockers.
• Maintain a torque and friction map for the joint family to ensure consistency across service actions.
• Archive inspection findings and replacement intervals to support continuous improvement and compliance.