1. Definition. 2. Sub-divisions of Theory of Machines. undamental Units. 4. Derived Units. 5. Systems of Units. 6. C.G.S. Units. P.S. Units. 8. M.K.S. Units 9. eBook free PDF download on Theory Of Machines by R. S. Khurmi, J.K. Gupta. Book download link provided by Engineering Study Material. Machine-Design-R-S-Kurmi R.S. KHURMI S. Chand & Company Ltd., especially to Mr. E.J preliminarysccl19 Khurmi, R. et al.; Theory of Machines, 14th ed.
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Notify me of new posts by email. Table of Contents: Related Posts Fundamentals of Machining Processes: Surface color[ edit ] A server grade flash memory card with a black heat sink. The heat transfer from the heat sink occurs by convection of the surrounding air, conduction through the air, and radiation.
Heat transfer by radiation is a function of both the heat sink temperature, and the temperature of the surroundings that the heat sink is optically coupled with.
In this case, finned heat sinks operating in either natural-convection or forced-flow will not be affected significantly by surface emissivity. In situations where convection is low, such as a flat non-finned panel with low airflow, radiative cooling can be a significant factor.
Here the surface properties may be an important design factor. Matte-black surfaces will radiate much more efficiently than shiny bare metal.
The emissivity of a material is tremendously frequency dependent, and is related to absorptivity of which shiny metal surfaces have very little. For most materials, the emissivity in the visible spectrum is similar to the emissivity in the infrared spectrum[ citation needed ]; however there are exceptions, notably certain metal oxides that are used as " selective surfaces ".
In a vacuum or in outer space , there is no convective heat transfer, thus in these environments, radiation is the only factor governing heat flow between the heat sink and the environment. Microprocessor cooling[ edit ] Cooling system of an Asus GTX graphics card; three heat pipes are visible Heat dissipation is an unavoidable by-product of electronic devices and circuits.
To ensure that the component does not overheat , a thermal engineer seeks to find an efficient heat transfer path from the device to the environment. The heat transfer path may be from the component to a printed circuit board PCB , to a heat sink, to air flow provided by a fan, but in all instances, eventually to the environment. This will be discussed under the section attachment methods.
For each interface between two objects in contact with each other, there will be a temperature drop across the interface. For such composite systems, the temperature drop across the interface may be appreciable. Attachment methods[ edit ] As power dissipation of components increases and component package size decreases, thermal engineers must innovate to ensure components won't overheat.
Devices that run cooler last longer. A heat sink design must fulfill both its thermal as well as its mechanical requirements.
Concerning the latter, the component must remain in thermal contact with its heat sink with reasonable shock and vibration. The heat sink could be the copper foil of a circuit board, or a separate heat sink mounted onto the component or circuit board. Attachment methods include thermally conductive tape or epoxy, wire-form z clips , flat spring clips, standoff spacers, and push pins with ends that expand after installing. Thermally conductive tape Roll of thermally conductive tape.
A pin fin heat sink with a Z-clip retainer. Wire form Z-clips More expensive than tape and epoxy, wire form z-clips attach heat sinks mechanically. To use the z-clips, the printed circuit board must have anchors. Anchors can be either soldered onto the board, or pushed through. Either type requires holes to be designed into the board. To assemble with a z-clip , attach one side of it to one of the anchors. Deflect the spring until the other side of the clip can be placed in the other anchor.
The deflection develops a spring load on the component, which maintains very good contact.
In addition to the mechanical attachment that the z-clip provides, it also permits using higher-performance thermal interface materials, such as phase change types.
Clips Available for processors and ball grid array BGA components, clips allow the attachment of a BGA heat sink directly to the component.
The clips therefore require no holes in the PCB. They also allow for easy rework of components. A pair of push pins. Push pins with compression springs For larger heat sinks and higher preloads, push pins with compression springs are very effective. The compression spring holds the assembly together and maintains contact between the heat sink and component. Care is needed in selection of push pin size. Too great an insertion force can result in the die cracking and consequent component failure.
Threaded standoffs with compression springs For very large heat sinks, there is no substitute for the threaded standoff and compression spring attachment method. One end is secured with a screw through a hole in the PCB. The other end accepts a screw which compresses the spring, completing the assembly. A typical heat sink assembly uses two to four standoffs, which tends to make this the most costly heat sink attachment design.
Another disadvantage is the need for holes in the PCB.
Summary of heat sink attachment methods  Method.