04 SCKKW 



from the loose to the fast pulley, and so puts the compress- 

 ing ram in action. Two belts are used with a quick return 

 motion, so that the compressing ram can be run out more 

 quickly than it is driven in. 



The adoption of one or another of these three systems of 

 hydraulic supply depends, to some extent, upon the exist i n- 

 conveniences ot the testing laboratory whether there is 

 shaft power available, whetner there is an abundant supply 

 ot low-pressure water, and so forth. Probably the three 

 systems might be placed in the following order, as regards 

 tneir convenience and adaptability : (1) Pumps and accu- 

 mulator ; (2) Intensifier ; (3) Screw compressor. In much 

 of the testing work carried out, speed and continuity of 

 action are of the greatest importance. This latter is lacking 

 in both intensiner and compressor, whose immediate 

 capacity is limited to the volume swept out by one stroke 

 of the ram. Nothing is more annoying than to have to 

 wait, either at the beginning of a test or, what is worse, in 

 the middle of one, until the ram has been brought back to 

 its initial position ready for another stroke. This is a thing 

 that may happen, especially when the specimen is a long 

 one of very ductile material, requiring a large move- 

 ment of the straining ram before fracture takes place, and 

 also when the cup leathers are a little leaky. 



On the other hand pumps can be kept continuously 

 running, and any excess of water coming from them simply 

 goes to the accumulator to wait until its use is required. 

 An objection sometimes raised against pumps is that they 

 give rise to a pulsating action on the specimen. This is to 

 some extent true, but it is a difficulty which can be met 

 by pumping up the accumulator before a test, stopping 

 the pumps during the test, and working only from the 

 accumulator ; as soon as the test is complete, the pumps 

 can be started and kept going during the interval preced- 

 ing the next test. Another objection to the pump system 

 is that in this case the supply of water to the straining 

 cylinder must be directly controlled, and, as the water 

 pressure gets as great as 2 or 3 tons per sq. in., it will 

 readily be understood that the valve must be under 

 perfect control, and in order to easily manipulate the 

 floating beam, the lever controlling the valve spindle 

 must be of large radius. Two consecutive valves are 

 better than one. 



The above remarks apply equally well to all kinds of 

 testing machines. 



