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Chapter 1
Introduction
1.1 Power Transmissions and Mechanical Drives
Mechanical power transmissions1 consist of units which, in distinction from electrical, pneumatic and hydraulic ones, transfer power from the prime mover to the actuator (operational machine or operational member) with the assistance of rotary motion. These units are called mechanical drives and are situated between the prime mover and the actuator (Figure 1.1). The drive is connected with both the prime mover and the actuator by couplings or clutches forming an entirety whose function is defined by the purpose of the actuator.
The embedding of a power transmission to link the prime mover and the machine operating member can be due to a number of reasons:
- The required speed of the machine operating member very often differs from the speeds of the standard prime movers.
- One prime mover has to drive several actuators.
- The driven side speed has to be frequently changed (regulated), whereas the prime mover cannot be used to full advantage for this purpose.
- Certain periods of the driven side operation may require torques far from those obtained on the motor shaft.
- As a rule, standard motors are designed for uniform rotary motion, while operating members have sometimes to move with varying speed or periodic halts.
- If a resonant vibration of some member in the chain of power transmission cannot be solved in any other way, the frequency of rotary motion can be changed by building-in a drive.
- Sometimes considerations of safety, convenience of maintenance or the dimensions of the machine, especially if the prime mover and operational machine shaft axes are not coaxial, do not allow the direct coupling of the prime mover shaft with operating member.
The capital task of the designer is to select such an assembly ‘prime mover – transmission (drive)’ which should optimally meet the needs of the operational machine or member. This act of choosing is a complex task, whose solution depends on: (i) accessibility of the energy source and its price, (ii) efficiency of the entirety of prime mover – transmission – operational machine, (iii) investment costs, (iv) operational machine features, primarily the (v) variability of its speed of rotation, (vi) service conditions, (vii) drive maintainability and so on. Within the framework of this task, a particularly complex problem is defining the transmission: mechanical or some other? This question is beyond the scope of this book, but generally it may be affirmed that the basic advantage of mechanical drives in relation to all the others is their very high efficiency, which is becoming more and more important day by day.
The comparative advantages offered by possible transmissions and drives are outlined in Table 1.1 which gives only a general illustration. Recently, a prominent feature in power transfer has been the extensive employment of electric, hydraulic and pneumatic transmissions. Frequently, such transmissions together with mechanical drives are simultaneously used to actuate various mechanisms. The proper choice of a drive for each specific case can be made only by comparing the technical and economical features of several designs.
Table 1.1 Advantages of transmissions and drives.
The mechanical drive driving shaft receives power P1 at speed of rotation n1 from the prime mover driven shaft, and the mechanical drive driven shaft supplies power P2 < P1 at speed of rotation n2 to the operational machine driving shaft. The difference P1 − P2 = PL is called power loss and the ratio:
is called efficiency; it takes a special place amongst power transmission characteristics because it shows unproductive power expenditure and so indirectly characterizes the wear of the drive and its warming up – the capital problems in power transmissions. Warming up causes strength and lifetime decrease of drive parts. Their corrosion resistance and the functional ability of lubricant are also imperilled. The importance of efficiency is raised to a power by the global lack of increasingly expensive energy and its value also decisively affects the price of the drive.
The power loss consists of constant losses which on the whole do not depend on load, and variable losses which on the whole are proportional to the load. The value of constant losses approximates the power of idle run, that is, the power needed to rotate the drive at P = 0 on the driven shaft. It depends on the weight of the drive parts, the speed of rotation and the friction in the bearings and on other surfaces of contact.
The second fundamental parameter of a mechanical drive is the transmission ratio i defined as the ratio of its driving n1 and driven n2 shaft speeds of rotation or angular speeds:
If i > 1 (n1 > n2) the mechanical drive is called an underdrive and its member is called a reducer. It reduces the speed of rotation and the transmission ratio is also called a speed reducing ratio. If i < 1 the mechanical drive is called an overdrive, its member is called a multiplicator and the transmission ratio is also called a speed increasing ratio. It multiplies the speed of rotation. An overdrive usually works less efficiently than an underdrive. This is especially true for a toothed wheel gearing.
1.2 Classification of Mechanical Drives
The basic division of mechanical drives falls into:
- Drives with a constant transmission ratio.
- Drives with a variable transmission ratio.
In constant transmission ratio drives, the constant speed of driving shaft rotation results in a constant speed of driven shaft rotation, n2 = n1/i. Their design should, as a rule, include at least the following data: (i) transmitted power of the driving (P1) or driven (P2) shaft or related torques, (ii) speed of rotation (rpm) of the driving (n1) and driven (n2) shaft, mutual location of the shafts and distanc...
