Mechanical Engg Simple Notes ,Solved problems and Videos

self locking property - torque required to lower the load, T= Wtan(φ - α)xd/2 self locking property of the threads-if φ > α the torque required to lower the the load will be positive, indicating that an effort is applied to lower the load. if friction angle is greater than the helix angle or coefficient of friction is greater than the tangent of helix angle(2marks) Over hauling of screwsin the above expression, if φ< α,then the torque required to lower the load will be negative.

(i) it is easier to overextend the extension spring. Compression springs will bottom out before the overextend. Also it seems like the tensile strength will be weaker at the attachment point for the extension spring, making it generally larger and more cumbersome to correct the deficiency

self locking property of the threads-if φ > α the torque required to lower the load will  be positive, indicating that an effort is applied to lower the load. if friction angle is greater than the helix angle or coefficient of friction is greater than the tangent of helix angle applications- for very large use of screw in threaded fastener, screws in screw top container lids, vices, C-clamps and screw jacks

(i) efficiency of screw η = tan α / tan(α +φ) And for self locking screws, φ ≥α or α≤ φ Efficiency ≤ tan(φ) /tan(φ +φ) ≤ tan φ/tan 2 φ ≤tan φ/ (2 tan φ/(1-tan2 φ)) ≤tan φ X (1-tan2 φ)/ (2 tan φ) ≤ ½-tan2 φ/2 From this expression efficiency of self locking screw is less than 50%

Each product is to be design to perform a specific function or a set of functions to the satisfaction of customers. In a present days of buyer’s market, with a number of products available in the market are having most of the parameters identical,the appearance of the product is often a major factor in attracting the customer. For any product, there exists a relationship between the functional requirement and the appearance of a product. The aesthetic quality contributes to the performance of the product, through the extent of contribution varies from product to product.

Causes-1) Bending failure-every gear tooth acts as a cantilever. If the total repetitive dynamic load acting on the gear tooth is greater than the beam strength of the gear tooth then the gear tooth will fail in bending remedies-module and face width of the gear is adjusted so that the beam strength is greater than the dynamic load 2)Pitting-surface fatigue failure which occurs due to many repetition of Hertz contact stresses , failure occurs when the surface contact stresses are higher than the endurance limit of the material.

Definition w.r.t. bolts 1) Major dia.- dia. Of imaginary cylinder parallel with the crest of the thread ,it is the distance from crest to crest largest dia. of an external or internal thread 2) Minor dia.-dia. Of imaginary cylinder which just touches the roots of an external thread or smallest dia.of an external or internal screw thread 3) Pitch-distance from a point on one thread to the corresponding point on the next thread. 4) lead- distance between two corresponding points on the same helix

Effect of keyways – when the keyways are cut on the shafts, material is removed at the skin, there by weakening the cross section of the shaft. Stress concentration effect is also serious at the corner of the keyways. Thus the shaft become weak. Type of key- Hollow saddle key or Tangent key

Definition of 1) Free length-it is a length of spring in unloaded condition 2) Solid height-it is a length of spring in fully loaded condition 3) Spring rate-load per unit deflection 4) Spring index- ratio of mean diameter of coil to diameter of wire

Composition in percentage 1) Carbon-0.3-0.4 %, manganese 0.5 % and molybdenum 2.8 % 2) Carbon-0.26-0.34, ,Nickel 1 % and Chromium 0.25 % 3) Carbon-0.2-0.3,chromium 0.75 % and molybdenum 5.5 %