Complete Guide to Bolt Calculation- Size, Grade, Preload and Thumb rules

Understanding Bolt Sizing in Mechanical Design

The Importance of Calculated Design

• The speaker emphasizes a calculated approach to mechanical design, particularly in bolt sizing, stating that while calculations are essential, experience and thumb rules often suffice.

• In many cases, such as mounting brackets for pneumatic cylinders, standard sizes provided by manufacturers can be used without detailed calculations.

When Calculations Are Necessary

• Critical situations require precise calculations; examples include hydraulic systems with heavy loads and high-torque drive lines subjected to dynamic loading.

• Continuous vibration or impact loading also necessitates careful bolt sizing to prevent failures. Engineers must calculate the right size and grade of bolts and specify optimum preload.

Overview of the Video Content

• The video serves as a comprehensive guide on bolt sizing, covering both calculation methods for critical situations and thumb rule methods for non-critical scenarios.

• By the end of the video, viewers will learn when to perform detailed calculations versus relying on simpler methods.

Understanding Bolting Connections

• The strength of a bolting connection is not solely dependent on bolt size; proper preload is crucial to prevent loosening under vibration.

• Bolting connections allow for non-destructive assembly/disassembly compared to welding or riveting. They can be executed using either bolts with nuts or screws directly into threaded components.

Choosing Between Bolting and Screwing Methods

• The choice between bolting (with nuts) and screwing (without nuts) depends on application constraints like space availability around the nut.

• For steel bolts in various materials, recommended thread engagement varies: 1x diameter for steel-to-steel, 1.5x for steel-to-soft materials, and 2–3x for very soft materials like plastic or wood.

Thread Engagement Insights

Understanding Bolt and Screw Connections

Importance of Thread Engagement

• The engaged thread length should be optimal; excessive length does not enhance strength but increases material waste and manufacturing costs.

• Clearance holes are generally more cost-effective than threaded holes, especially in heavy structures requiring numerous fastenings.

Introduction to Ziqual CNC Manufacturing Service

• Ziqual simplifies the process of sourcing parts by allowing users to upload CAD files for quotes from trusted manufacturers globally.

• Unique features include a digital recipe (name plate archive) for reordering parts easily via QR code, ensuring consistency in quality and design.

Mechanism of Bolt Tightening

• Proper tightening involves applying preload after the bolt head contacts the component; stopping here can lead to loosening under load.

• Preload is essential to prevent separation under axial loads and leakage in applications like flange seals.

Calculating Preload and Torque

• Preload should be approximately 70% of the bolt's yield strength; this can be calculated using specific formulas based on bolt size and grade.

• For an M10 grade 8.8 bolt, calculations show a preload force around 26 kN, which translates into a tightening torque of about 52 Nm.

Challenges with Torque Method

• The torque method has accuracy issues due to friction losses; critical applications often require direct measurement methods for preload.

• Understanding that external tensile loads do not directly transfer to bolts until preload is overcome helps reduce fatigue failure risks during vibrations or impacts.

Benefits of Preloading Bolts

• With proper preload, only part of the external load affects the bolts, enhancing their lifespan under cyclic stress conditions.

Understanding Bolt Preloading and Washer Functions

The Importance of Bolt Preloading

• Preload does not increase the strength of the bolt itself but enhances the reliability of bolting joints by preventing loss, reducing fatigue, and avoiding direct shear or bending stress on bolts.

• Washers do not increase the load-carrying capacity of bolts; instead, they serve to distribute loads over a larger area, which is crucial for softer materials like aluminum or brass.

Functions of Washers in Bolting

• Washers help protect surfaces during tightening by absorbing wear that would otherwise affect joint materials when nuts or bolt heads rotate against them.

• They improve frictional locking by providing a smoother contact area, which helps prevent loosening under vibration, especially in uneven surfaces.

Understanding Bolt Sizing Calculations

• A bolting connection can fail if the applied load exceeds the ultimate tensile strength of the bolt. However, yield strength is used as a design limit since exceeding it leads to permanent stretching and loss of preload.

• To size a bolt correctly, calculate its cross-sectional area using force divided by stress (considering yield strength), factoring in safety margins for both tensile and shear loading.

Steps for Determining Bolt Size

• When determining bolt size, you can either select the number of bolts first or choose a specific bolt size based on constraints like plate thickness.

• For example calculations: If using N bolts, each must have an area equal to total required cross-section divided by N. Alternatively, if choosing a specific size first, calculate how many are needed based on their individual areas.

Example Calculation for Load Attachment

• In an example involving attaching a hook to a wheel loader capable of pulling 20 tons (20,000 kg), convert this load into Newton force before proceeding with calculations.

• Selecting the right grade for bolts is essential; grades classify strength according to international standards with different numbering systems indicating tensile and yield strengths.

Understanding Bolt Grades

• ISO metric grades are commonly used in machinery; they range from 8.8 to 10.9 where higher numbers indicate greater strength.

Understanding Bolt Strength and Selection

Key Parameters of Bolt Strength

• Proof Stress: The maximum stress a bolt can endure without permanent deformation, typically set at 90-95% of yield strength.

• Yield Strength: The stress level where bolts begin to deform permanently; crucial for design calculations.

• Tensile Strength: The ultimate stress before breaking; not used for design as it indicates failure.

Choosing the Right Bolt Grades

• Low Carbon Steel Grades (<8.8): Used in light applications like covers and panels due to lower manufacturing costs.

• Grade 8.8: Medium carbon steel that is quenched and tempered, balancing strength and toughness; commonly used in machinery.

• High Strength Grades (10.9 & 12.9): Alloy steels for high load applications, such as automotive suspension (10.9) or aerospace fixtures (12.9).

• Stainless Steel Bolts: Two grades are A2-70 (700 MPa yield strength) and A4-80 (800 MPa yield strength), often used in pharmaceutical industries.

Factors of Safety in Bolt Sizing

• For static joints, a factor of safety should be between 1.25 to 2 times the load; for cyclic loads, it increases to 2 to 3 times.

• In safety-critical parts, a factor of safety should range from four to eight times the expected load.

Calculating Required Bolting Area

• To support a load of 20 tons under yield limit, the required bolting area is calculated as 919.68 mm².

• Using four bolts results in an individual bolt area requirement of approximately 229.9 mm² each.

Determining Bolt Diameter

• The diameter calculation uses the formula for circular cross-section areas leading to an initial root diameter estimate of about 17.1 mm.

• Opting for M20 bolts ensures adequate safety margin over M18 based on calculated requirements.

Impact of Thread Pitch on Strength

• Fine thread pitches increase root diameter slightly compared to coarse threads, enhancing loading capacity but primarily serve better adjustment purposes in specific applications like sensors.

Dynamic Loading Conditions with Shear Stress

Designing for Cardan Shaft Applications

• A cardan shaft transfers torque between non-collinear shafts using universal joints; critical for handling radial misalignments.

Torque and Shear Force Calculations

• Given torque is converted into shear force using radius measurements from flange designs; this leads to calculating static forces acting on bolts.

Understanding Moment of Inertia

Angular Acceleration and Torque Calculations

Angular Acceleration Calculation

• The angular acceleration is derived from the known deceleration time of 0.5 seconds, indicating a rapid stop when brakes are applied.

• Angular velocity is given as 10,000 RPM, which converts to approximately 1046.7 radian/s using the formula omega = 2pi n/60 .

• The calculated angular acceleration results in a value of 2093 rad/s².

Torque and Force Conversion

• Inertial torque is computed to be 69.2 Newton m.

• This torque translates into a force of 117 Newton when divided by the radius.

• The total maximum static plus dynamic force on the bolt amounts to 14,020 Newton.

Bolt Selection and Shear Strength

Bolt Grade Selection

• An 8.8 grade bolt is selected with a yield strength of 640 MPa; however, shear strength must be considered for this application.

• The minimum shear yield strength is calculated as approximately 371 MPa (0.58 times tensile yield strength).

Required Bolting Area

• Required bolting area is determined by dividing force by shear stress multiplied by a factor of safety (5), resulting in an area requirement of 188.8 mm².

Bolt Sizing and Preloading

Bolt Size Consideration

• Due to space limitations in the flange, M8 or M10 bolts are chosen; starting with M8 bolts having a root diameter around 6.82 mm.

• Each M8 bolt has an area of about 36.6 mm²; thus, at least six bolts are required based on total area calculations.

Importance of Proper Preloading

• Plane washers should be used for load distribution; correct preloading for M8 bolts requires tightening torque around 25 Newton meters.

Thumb Rules for Bolt Sizing

General Guidelines

• Emphasis on practical thumb rules over calculations:

• For no-load connections like mounting plates, select proportionate bolt sizes without calculations (e.g., M6 or M8).

• Ensure sufficient wall thickness remains after counter-bolting (at least 1 or 2 mm).

Plate Thickness Considerations

• Thread engagement length should equal at least one bolt diameter for steel plates; aluminum requires engagement lengths between 1.5 to 2 times the diameter.

Load Estimation and Sheet Metal Guidelines

Load Capacity Insights

• A single M6 grade bolt can handle approximately one ton of tensile load while an M10 can manage around three tons—allowing simplified sizing under light loads (100–200 kg).

Recommendations for Thin Sheets

• Avoid threading if sheet metal thickness is less than two mm; use weld nuts or through holes instead.