Last time we saw how the involute profile of spur gear teeth ensures smooth contact between gears when they rotate. Today we’ll see why it’s important to be able to change the rotational speed of the driven gear in relation to that of the driving gear by modifying their gear ratio, the speeds at which gears move relative to one another. Why would we want to modify the rotational speeds of gears relative to one another? One reason is to compensate for the fact that alternating electric current (AC) motors drive most modern machinery, and these motors operate at a fixed speed determined by the 60 cycles per second frequency of electricity provided by the utility power grids of North America. By fixed speed I mean that the motor’s shaft revolves at a single, fixed rate. It can’t run any faster or slower. This is fine for some motorized applications, but not others. Basic machinery such as wood cutting saws, grinders, and blowers function well within the parameters of the AC motor’s fixed speed, because their working parts are intended to rotate at the same rate as the motor’s shaft. As a matter of fact, in this instance there’s often no need for a gear train, because the working parts can be connected directly to the motor’s shaft, and the machinery will be powered and function correctly. There are many instances however in which a fixed speed does not match the speed required for more complex machinery to correctly perform precise, specialized tasks. Take a machine tool meant to cut steel bars, for example. It has a rotating part meant to cut through the steel during machining, and to properly do so its cutting tool bit must turn at 400 revolutions per minute (RPM). If it turns any faster, the cut won’t be smooth and the tool bit will overheat and break due to increased friction. If the AC motor driving the machine tool turns at 1750 RPM, a common speed for such motors, then the tool bit will be turning at a much faster rate than the desired 400 RPM, and this presents a problem. To solve the problem we need only add a gear train between the motor and the part containing the tool bit, meaning, we must connect the gear train’s driving gear to the motor’s shaft and a driven gear to the part’s shaft. But in order for this arrangement to work a conversion must take place, that is, we must design the gear train to operate at a specific gear ratio. By gear ratio, I mean the speeds at which the two gears will rotate relative to one another. Next time we’ll introduce the gear ratio formulas that make it all work.
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Posts Tagged ‘motors’
When Do You Need To Modify Gear Ratio?
Wednesday, February 19th, 2014Systems Engineering In Medical Device Design – Preproduction, Part I
Monday, February 4th, 2013 If you’ve been following along with our blog discussion on the systems engineering approach to medical device design, you should by now be convinced that instructions are important. In fact, the meticulous instructions produced during the manufacturing, operating, and maintenance phases of the Development stage are also crucial to later stages, that of Production and Utilization. Let’s finish up our discussion on the Development stage by taking a look at its final aspect, Preproduction.
The Preproduction aspect is instrumental to nipping potential problems in the bud before the medical devices go into actual production. In the initial Preproduction stages, systems engineers coordinate with the manufacturing and purchasing departments within the company as well as outside suppliers. The goal is to acquire all parts and equipment necessary to build a limited number of medical devices on the assembly line. Subjects such as preference in molded plastic components, motors, gears, pumps, springs, electronic components, circuit boards, wire, and tubing are discussed and agreed upon. Vendors are assessed with regard to their ability to produce parts when they are needed and that meet design specifications, satisfy quality requirements, and have costs that fall within budgetary constraints. The assembly of Preproduction devices provides an opportunity for systems engineers to validate manufacturing and quality control instructions and assess the device design with regard to manufacturability, meaning, the extent to which devices can be manufactured with relative ease, at minimal cost, while maintaining maximum reliability. Devices manufactured during this aspect of the Development stage serve as a test. Are instructions clearly written? Do the device parts fit together as they should? Are parts strong enough to withstand the assembly process? Can the devices be assembled as quickly and easily as expected? If the answer is “no” to any of these questions, then the device design and instructions must be returned to the design engineers and technical writers. Heads come together to rehash things and work out the bugs. Next time we’ll continue with the Preproduction aspect of the Development stage to see how laboratory and field testing enables systems engineers to shake out any more bugs from the medical device design, operating instructions, and maintenance instructions. ___________________________________________ |
Transistors – Digital Control Interface, Part I
Monday, June 18th, 2012 In the navy, the captain is the brains behind a ship’s operations. He gathers information, makes important decisions, then issues orders. He’s not there to roll up his sleeves and swab the decks. The captain relies on the ship’s officers to act as an interface between himself and the sailors that perform the physical labor required on deck.
In this article we’ll see how the FET, that is, the field effect transistor, performs much the same role as the ship’s officers when it is used within electronic controls. There it acts as an interface between electronic components that issue commands and the electrical devices that carry them out. Last week we became familiar with field effect transistors and how their control of electrical current flow is analogous to how a faucet controls the flow of water. Although FETs can be used to vary the flow of current, they’re usually employed to perform a much simpler task, that of simply turning flow on or off, with no in-between modality. Like the captain of a ship, microprocessor and logic chips are the brains behind the operation in all sorts of industrial and consumer electronics. Figure 1 shows a few of them. Figure 1
The chips, which operate on low voltage, contain entire computer programs within them that gather information, make decisions, then instruct the higher voltage devices like motors, electrical relays, light bulbs, and audible alarms to follow. By “information,” I mean data signals received by the chip from its input connections to sensors, buttons, and other electrical components. This data informs the chip’s computer program of important operational information, like whether buttons have been pressed, switches are activated, and temperatures are normal. Based on this data, “decisions” are made by the chip using the logic contained within its program, then, depending on the decisions made, “commands” are issued by the chip. The commands, in the form of electrical output signals, are put into action by the work horses, the higher voltage devices. They, like a ship’s sailors, perform the actual physical work. There is one problem presented by this scenario, however. The electric output signals from the lower voltage chips are not suited to directly control the higher voltage devices because the signal voltage put out by the chips is too low. Even if the chip was designed to work at a higher voltage, the high level of current drawn by the motors, relays, and bulbs would lead to damage of the delicate circuitry within the chip. The chips must therefore rely on the FET to act as a digital control interface between them and the higher voltage devices, much as the ship’s captain depends on his subordinates to carry out his orders. Next week we’ll look at a real life example of how a digital interface is put into operation within an industrial product.
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