The next application on our list, pressure tank pumps in connection with elevators, represents a somewhat smaller percentage of H. P. hours of actual service in the motor as compared with the possible H. P. hours than in the case of an open tank pump. In case of the pressure tank the water reserve is usually limited, and the motor therefore must be equal to the continuous operation of the elevator at maximum load. Taking this fact into consideration, and the circumstances of elevator use being about the same in this case as in the case of the open tank elevator, we have a greater ratio of difference between the possible or contract H. P. hours in the motor and the H. P. hours of actual service, the maximum average use being about 70 per cent. to 75 per cent. and the minimum as low as 35 per cent. to 40 per cent., depending, of course, on the character of building in which the elevator is employed or the character of service. In calculating the size of motor required on an elevator of this description, a very convenient fact to remember is that every pound of pressure per square inch is equivalent to lifting water about 23 ft., or about 230 ft. per 100 pounds pressure, By reducing the required pressure to a relative lift in feet, and knowing the amount of water required by the elevator per minute, the motor calculation becomes the same as in case of the open tank elevator, the same allowances being made for friction, etc., as in the first case. The regulation of the motor in this case should be accomplished by the conditions of pressure in the pressure tank, as is the case with a steam pump employed in this service.
The next application of importance on the list is sewing machines. In the tests I have been able to make on this class of work I have obtained some singular results. One item of importance is the fact that the single thread machines, which are lightest running, consume the most power in operating. Paradoxical as this may seem, it is easily explained. As a rule this class of machine is used on light work, such as shirts, ladies' underwear, etc., and operated at a higher speed than any other class of machine. At equal speed the volts consumed on a single thread machine as compared with a shuttle machine is about as 2 to 3. In average commercial use, however, the positions are reversed, and the ratio of volts consumed in the single thread as compared with the shuttle machine is about as 5 to 3. To double the speed on a sewing machine requires about 2½ times the power. The difference in volts consumed on the different makes of sewing machines is so small that we may disregard it entirely, as well as the character of work done by the machine, for the heavier the work the slower the speed, and more frequent and longer stops on the machine, thus keeping the average volts per operator about constant in all cases. This leaves the speed in stitches per minute at the sewing machine the factor from which we must calculate the power required in a sewing machine plant. To illustrate this I will give you the record of two cases which are about the average. Case No. 1 is a shop in which are 30 sewing machines connected to a 2 H. P. motor. At the time tests were made there were but twenty operators at work, leaving ten idle machines, the entire shafting, however, being in operation. The class of goods manufactured in this shop is a cheap grade of cotton and wool pants, rather heavy goods to sew. A volt meter across the terminals of the motor gave the following readings with the current at 9 amperes: Minimum 90 volts, maximum 148 volts, average 119, which gives us a minimum average per operator of 4.5 volts and a maximum average of 7.4 volts, or a general average of 5.9 volts per operator. This motor was driving the shafting for 30 machines, and as the average operators employed the year round will not exceed 75 per cent. of the shop capacity, it will, I think, be entirely fair to estimate the average volts per machine rather than per operator, as the user of the motor has contracted for power sufficient to drive his entire plant. In this case, then, we have a minimum average of 3 volts per machine and a maximum of 4.9 volts, or a general average of say 4 volts per machine. A 2 horse motor of 82 per cent. efficiency with 9 amperes of current will require about 200 volts to develop 2 actual H. P. Two hundred volts therefore is what the electric light company contract to deliver, while, in reality, they deliver only 129 volts or 60 percent., or a minimum average of 90 volts or 45 per cent. of the power contracted for. These machines were making about 1,200 stitches per minute--an average of 4 volts per 100 stitches.