Mechanical Design Handbook

JOINT DESIGN AND FASTENER SELECTION Joint Length The longer the joint length, the greater the total elongation will occur in the bolt to produce the desired clamp load or preload. In design, if the joint length is increased, the potential loss of preload is decreased. Joint Material If the joint material is relatively stiff compared to the bolt material, it will compress less and therefore provide a less sensitive joint, less sensitive to loss of preload as a result of brinelling, relaxation, and even loosening. Thread Stripping Strength Considering the material in which the threads will be tapped or the nut used, there must be sufficient engagement length to carry the load. Ideally, the length of thread engagement should be sufficient to break the fastener in tension. When a nut is used, the wall thickness of the nut as well as its length must be considered. An estimate, a calculation, or joint evaluation will be required to determine the tension loads to which the bo...

All design activities must do the following: Know the “customers’ needs.” Define the essential problems that must be solved to satisfy the needs. Conceptualise the solution through synthesis, which involves the task of satisfying several different functional requirements using a set of inputs such as product design parameters within given constraints. Analyse the proposed solution to establish its optimum conditions and parameter settings. Check the resulting design solution to see if it meets the original customer needs. The Iterative Nature of Design Design proceeds from abstract and qualitative ideas to quantitative descriptions. It is an iterative process by nature: new information is generated with each step, and it is necessary to evaluate the results in terms of the preceding step. Thus, design involves a continuous interplay between the requirements the designer wants to achieve and how the designer wants to achieve these requirements. Designers often...

The Heart of Automation: Mechanical Cam Design Classes of Cams Cams may, in general, be divided into two classes: uniform motion cams and accelerated motion cams. The uniform motion cam moves the follower at the same rate of speed from the beginning to the end of the stroke. However, as the movement starts from zero to full speed instantly and stops in the same abrupt way, there is a distinct shock at the beginning and end of the stroke if the movement is at all rapid. In machinery working at a high rate of speed, therefore, it is important that cams are so constructed that sudden shocks are avoided when starting the motion or when reversing the direction of motion of the follower. The uniformly accelerated motion cam is suitable for moderate speeds, but it has the disadvantage of sudden changes in acceleration at the beginning, middle, and end of the stroke. A cycloidal motion curve cam produces no abrupt changes in acceleration and is often used in high-speed machinery beca...

A flywheel is a mechanical device with significant moment of inertia used to store rotational energy . Flywheels resist changes in rotational speed, helping to steady shaft rotation when a fluctuating torque is applied by its power source, such as a piston-based (reciprocating) engine, or when an intermittent load, such as a piston pump, acts on it. Flywheels can also produce high-power pulses for experiments, where drawing power from the electrical network would create unacceptable spikes. A small motor can accelerate the flywheel between pulses. Flywheels may be classified as balance wheels or flywheel pulleys . Their primary purpose is to equalize energy exertion and work done, thus preventing excessive or sudden speed changes. Permissible speed variation is a key factor in all flywheel designs: for example, steam engines may allow only 1–2% variation, whereas punching or shearing machinery may tolerate 20% variation. The function of a balance wheel is to absorb...

In practice, most machines involve rotary motion as well as linear motion. Typical examples include electric motors, gears, pulleys, flywheels, and internal combustion engines. If we wish to calculate how quickly a machine reaches its full operating speed —in other words, determine the acceleration of its components— we must consider rotary acceleration and the associated torques, in addition to linear acceleration. Fortunately, Newton’s second law of motion applies equally well to rotary motion, provided that the correct rotational form of the equation is used. Consider a solid disc mounted on a shaft and rotated by a pull cord wrapped around its rim. In this case, we cannot apply the standard linear form of Newton’s second law, F = ma , because although a linear force is applied through the tension in the cord, the resulting motion is rotational rather than linear. There is no single straight-line acceleration that can be expressed in m/s 2 . Furthermore, not all parts of t...

Let us return to the legend of Newton and the falling apple. From the study of statics, we know that the apple remains attached to the tree as long as the apple stalk is strong enough to support the weight of the apple. As the apple grows, there comes a point where its weight becomes too great for the stalk to withstand. When this happens, the stalk breaks and the apple falls. The physical quantity that increases as the apple grows is its mass. Mass is often confused with weight, but the two are fundamentally different. There have been many experiments involving fruits and seeds grown inside orbiting spacecraft. In such an environment, objects are effectively weightless and would float freely if not restrained. Despite this, the fruits still possess mass. Mass is the amount of matter contained in a body and is measured in kilograms (kg). The reason the apple hangs downward on the tree, and eventually falls downward, is due to a force of attraction between the Earth and any...

When Newton first published his laws of motion in the seventeenth century, they caused significant controversy. Even the leading scientists and mathematicians of the time struggled to understand his ideas, and very few were able to follow his reasoning. Today, these concepts are much easier to grasp because we are familiar with ideas such as gravity, acceleration, and inertia. We observe them through everyday experiences such as watching astronauts float in space, satellites orbit the Earth, or even by feeling acceleration and deceleration during roller-coaster rides at amusement parks. Isaac Newton lived during the latter half of the seventeenth century and was born into a prosperous family in Lincolnshire, England. He demonstrated exceptional intellectual ability at a young age and was appointed Professor at Cambridge University at the remarkably early age of 21. Newton devoted much of his life to investigating astronomy, optics, and heat. However, he is best remembered ...

In a design situation, the expected load on a column is known along with the required column length. The designer must specify the following parameters: 1. The manner of attaching the column ends to the structure, which affects end fixity. 2. The general shape of the column cross section (round, square, rectangular, hollow tube, etc.). 3. The material selected for the column. 4. The design factor appropriate for the application. 5. The final dimensions of the column. It is often desirable to propose and analyze several alternative designs to reach an optimum solution. Design software significantly facilitates this iterative process. It is assumed that for each design trial, the designer specifies items 1 through 4. For simple cross sections such as solid round or square shapes, the final dimensions can be computed using classical buckling equations such as the Euler formula or the J. B. Johnson formula. If a closed-form algebraic solution is not possible, iterative calc...

Reactions are the forces and/or couples acting at the supports and holding the beam and holding the beam in place. In some cases the user should enter a distributed load to account for the weight of the beam. The shear V effective on a section is the algebraic sum of all forces acting parallel to and on one side of the section, The bending moment is the algebraic sum of the moments due to applied loads and other applied moments to one side of the section of interest. Using value V bending moment can be calculated where x = position on the beam measured along its length M 0 = constant of integration evaluated from the boundary conditions. A bending moment that bends a beam convex downward (tensile stress on bottom fiber) is considered positive, while convex upward (compressive on bottom fiber) is negative. Moment and shear diagram constructed by plotting to scale the particular entity as the ordinate for each section of the beam. Such diagrams show in continuous form the...

Bolt Selection for Required Clamping Force A set of n bolts is used to provide a required clamping force   F between two mechanical components. The total load is assumed to be distributed equally among all bolts. The clamping load carried by each bolt is: P = F / n Each bolt is selected from an SAE graded steel and is designed to operate at K% of its proof strength. The parameter K is known as the demand factor . If the selected bolt material has a proof strength σ (psi), the allowable tensile stress is: σ a = K × σ The minimum required tensile stress area of the bolt is then: A t = P / σ a Using standard thread tables, a bolt size is selected with a tensile stress area equal to or greater than A t . Tightening Torque Requirement The tightening torque required to achieve the desired clamping load is estimated by: T = k 1 × D × P where: D = nominal outside diameter of the thread P = clamping load per bolt k 1 = torque co...