Design of a C – Clamp Asanga Ratnaweera Dept of Mechanical Engineering ME 209 Machine Design I Design of a C – Clamp Asanga Ratnaweera Dept of Mechanical Engineering

Introduction to Clamps Some commonly used clamps C-clamp Pipe clamp Hand screw clamp Quick action clamp Miter clamp 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering C - Clamp Some examples 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Engineering Design Process Functional requirement -> Design 1. Conceptualization 2. Synthesis 3. Analysis 4. Evaluation 5. Representation 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of a C - Clamp Identification of component Collar Nut Screw Handle Screw Head Frame 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of a C – Clamp Prevention of Failure Decides the size of each component considering its safety under the maximum permissible load conditions Other Design Requirements Ergonomics, Standards, Operational Parameters. 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Modes of Failure Compressive failure Tensile failure Shear failure Bending failure Torsion failure Fatigue failure Many more……. Note : it is important to identify the maximum stress and its location for each element under a given load condition 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Stress analysis Note 1 : Identification of actual load condition and the area is extremely important Note 3 : Bending and torsional stresses are analyzed using the following equations Note 2: Direction of the load applied will determine the type of the stress Bending Torsion 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Factor of safety Also know as safety factor is a multiplier applied to the calculated maximum stress to which a component will be subjected. Typically, for components whose failure could result in substantial financial loss or serious injury or death, high safety factor is used. 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Screw Treads The principal uses of threads are: for fastening, for adjusting, and for transmitting power 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Standard Thread Systems SI (ISO) Unified or American ACME Pipe Whitworth (BSW) 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Typical Screw Designation In ACME system 1/2” - 13 UNC - 2A external thread (B means internal) Class of fit (1 is loosest tolerance, 3 is tightest) Thread Series UNC (Unified Coarse) UNF (Unified Fine) Pitch (threads/inch) Nominal Diameter (also shown as decimal or screw #) 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Typical Screw Designation In Metric system a screw is designated by the nominal size (major diameter) and pitch being separated by the sign X. e.g. M8 X 1. M1.6 X 0.35 M2 X 0.4 M2.5 X 0.45 M3 X 0.5 M3.5 X 0.6 M4 X 0.7 M5 X 0.8 M6 X 1 M8 X 1.25 M10 X 1.5 M12 X 1.75 M14 X 2 M16 X 2 M20 X 2.5 M24 X 3 M30 X 3.5 M36 X 4 M42 X 4.5 M48 X 5 M56 X 5.5 M64 X 6 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Power Screws Power Screws are linear actuators that transform rotary motion into linear motion. Power screws are generally based on ACME , Square, and Buttress threads.   ACME 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Power Screws Square Buttress 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Power Screws Ball screws are a type of power screw.  7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Power Screws Efficiencies of between 30% and 70% are obtained with conventional power screws.   Ball screws have efficiencies of above 90%. Power Screws are Used for the following three reasons To obtain high mechanical advantage in order to move large loads with minimum effort. e.g Screw Jack. To generate large forces e.g A compactor press. To obtain precise axial movements e.g. A machine tool lead screw. Familiar applications clamps or vises, presses, and jacks. 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Screw Threads Wrapping an inclined plane around a cylinder results in a screw threaded or power screw. 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Screw Threads What is the thread? The raised helical rib going around a screw What is the tread angle (λ)? The tread angle is the angle of the inclined plane. The more threads you have the smaller the angle. The less thread you have the bigger the angle. Lead = pitch * number of starts 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Screw Threads If one turn of a square thread is unwrapped, the following ramp can be obtained. L dm – mean diameter dm 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Forces on Screw Threads Identical to pushing an object up along a thread 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Forces on Screw Threads Considering a small element of the nut δP - small force on the element due to the torque, δW - a small part of the load which the element is supported, δN - the normal force, δF - the friction force λ δW δN δF δP Motion 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Forces on Screw Threads Considering the force equilibrium, when the screw is about to rotate λ δW δN δF δP Motion 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Forces on Screw Threads However, the friction angle φ; The torque required Therefore, the total torque; Rm – mean radius 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Swivel Head Rc F Torque required to turn the collar 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of the Nut Shear and bearing failure of threads Bearing area screw nut Shear area 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Drawing of Screw Threads 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of the frame The frame is subjected to combined bending and direct stresses W Wx W W x 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of the frame An I-section can be used for the frame P σ t 6t 9t 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of a C - Clamp Major Design Steps 1. Calculate the torque required at the handle To overcome the friction at the screw To overcome the friction at the collar 2. Calculation of stress on the screw Maximum torsional stress Maximum direct stress 3. Bearing pressure on threads 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering

Asanga Ratnaweera, Department of Mechanical Engineering Design of a C - Clamp Major Design Step 4. Selection of number of starts 5. Check the shear failure of threads 6. Calculation of the stresses on the frame Moment due to couple Direct stresses 7 December 2018 Asanga Ratnaweera, Department of Mechanical Engineering