A mechanical system is a system of elements that interact on mechanical principles. Manages power to accomplish a task that involves forces and movement. Modern machines are systems consisting of (i) a power source and actuators that generate forces and movement, (ii) a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement, (iii) a controller with sensors that compare the output to a performance goal and then directs the actuator input, and (iv) an interface to an operator consisting of levers, switches, and displays.
This can be seen in Watt's steam engine (see the illustration) in which the power is provided by steam expanding to drive the piston. The walking beam, coupler and crank transform the linear movement of the piston into rotation of the output pulley. Finally, the pulley rotation drives the flyball governor which controls the valve for the steam input to the piston cylinder.
The adjective "mechanical" refers to skill in the practical application of an art or science, as well as relating to or caused by movement, physical forces, properties or agents such as is dealt with by mechanics. Similarly Merriam-Webster Dictionary defines "mechanical" as relating to machinery or tools.
Power flow through a machine provides a way to understand the performance of devices ranging from levers and gear trains to automobiles and robotic systems. The German mechanician Franz Reuleaux wrote, "a machine is a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motion." Notice that forces and motion combine to define power.
More recently, Uicker et al. stated that a machine is "a device for applying power or changing its direction." McCarthy and Soh describe a machine as a system that "generally consists of a power source and a mechanism for the controlled use of this power."
Example - multi-story house[edit | edit source]
A multi-story house is held at the top of a square shaft 5 meters by 5 meters by 100 meters down into the Earth. Explosive bolts hold the house at the top of the shaft. The house is square, about 4.9 meters by 4.9 meters and 12 meters tall. When detectors on the roof detect photons from a nearby asteroid hit or H bomb explosion: The bolts blow, and the entire house begins to descend into the shaft at 16 meters per second per second. Since this is faster than gravity, very short burn rockets are needed on the roof to help push the house downward for 1/4 second. The house decends 1/2 meter and the loose items in the house, including the people are levitated to about 1/5 meter off the floor. Next, the house descends at 9.5 meters per second = about 99% of the acceleration due to gravity, for 2 seconds. Lesser rockets will be needed to maintain this acceleration due to air resistance. The house descends 19 meters for a total descent of 19.5 meters. This means the house is clear of the top of the shaft, so the blast doors can finish closing about 1/10 th second later at t = 2.35 seconds. The loose stuff and people have returned to the floor. We next allow the people 1/10 th second to recover their balance t = 2.35 seconds at normal gravity, but the house is falling, so we need to hold the decent rate constant at about 23 meters per second to produce the sensation of not falling. The average acceleration for the first 2.25 seconds was about 12 meters per second so the speed of falling is 27 meters per second. From t= 2.25 to 2.35 the house falls 2.7 meters to a depth of 22.2 meters. We now need to decelerate the house to zero with respect to the bottom of the shaft. If we decelerate at 9 meters per second, deceleration time is 3 seconds and the people experience almost twice normal gravity during the 3 seconds. The house decends another 40.5 meters during the last 3 seconds to t = 5.35 seconds and 62.9 meters down the shaft. We should likely think about 100 meters as a safety factor, to allow gradual transitions and the possibility that the blast will accelerate the house down the shaft if it arrives before the blast doors fully close, or the blast doors fail due to the blast.
Advantages[edit | edit source]
Only 2.25 seconds warning of the blast are needed and the shelter is the house the family lives in.
Disadvantages[edit | edit source]
The house is small and the shaft may cost more than a million dollars to construct. An extra room is needed at the bottom of the shaft for survival supplies and equipment. A sloping tunnel is needed to the surface when it is safe to return to the surface.
References[edit | edit source]
- Oxford English Dictionary
- Merriam-Webster Dictionary Definition of mechanical Archived 2011-10-20 at the Wayback Machine.
- Reuleaux, F., 1876 The Kinematics of Machinery Archived 2013-06-02 at the Wayback Machine. (trans. and annotated by A. B. W. Kennedy), reprinted by Dover, New York (1963)
- J. J. Uicker, G. R. Pennock, and J. E. Shigley, 2003, Theory of Machines and Mechanisms, Oxford University Press, New York.
- J. M. McCarthy and G. S. Soh, 2010, Geometric Design of Linkages, Archived 2016-08-19 at the Wayback Machine. Springer, New York.