Nov. 27, 1884] 



NA TURE 



85 



Through the piston there is a small hole, D, called the 

 leak-hole, this being one of the main features of the 

 brake, the mode of action of which is as follows : — 

 A vacuum is created in the continuous pipe by means 

 of the ejector on the engine, the air being drawn from 

 below the pistons in the brake-cylinder by the branch 

 connections ; the air in the reservoir C leaks through the 

 leak hole D, and after a short time there is an equal 

 vacuum above and below the pistons. The brake is now 

 charged, and in its usual condition when the train is 

 running, the vacuum being maintained against acccidental 

 leakage by the continual use of the small ejector. 



To apply the brake, air is admitted into the continuous 

 pipe, destroying the partial vacuum, and, increasing the 

 pressure below the pistons, causes them to rise, breaking at 

 the same time the air-tight joint made by the piston against 

 the packing ring, thereby admitting air direct, through 

 the gland, into the lower part of the brake-cylinder, causing 

 the application of the brakes to be nearly instantaneous. 

 It will be observed that, directly the piston is forced up by 

 the atmospheric pressure, the vacuum in the reservoir 

 will gradually be destroyed by air passing through the 

 leak-hole, in fact after less than two minutes it has leaked 

 itself entirely off. It is also evident that it cannot be 

 instantly charged, for the vacuum in the reservoir has to 

 be created through the leak-hole. 



It is stated by some that the Midland automatic vacuum 

 answers all the Board of Trade conditions, and is there- 

 fore to be regarded as an effective serviceable automatic 

 brake. On studying the reports in the returns, and the 

 failures of this brake as reported in the technical papers, 

 we see how absurd the claim to efficiency becomes. For 

 example, the brake cannot be applied quickly several 

 times in succession ; when applied even once, the effective 

 brake power has all vanished in two minutes, thus getting 

 the doubtful name of the " two-minute leak-off brake.'' 

 Again, suppose a train became divided from any cause, 

 when ascending a heavy gradient, the bra'ce should auto- 

 matically apply itself, and remain applied until taken off 

 by hand. \Yha,t would the Midland automatic vacuum 

 do under the above circumstances ? Certainly the brake 

 would apply itself, but in two minutes or less all the 

 available brake power will have vanished, and, should the 

 hand-brake in the rear van not prove powerful enough to 

 hold the train on the bank, it will commence to run back. 



Although the Midland Company have the automatic 

 vacuum in general use, it is no criterion that the brake is 

 satisfactory ; we have only to add that the engines and 

 tenders are fitted with an efficient steam brake, so that in 

 entering stations, should the automatic vacuum fail, the 

 steam brake is quite capable of stopping the train, only 

 taking a little further distance to pull up in. At terminal 

 stations sometimes this is very awkward, as the accident at 

 the Liverpool Central Station, which happened some time 

 ago, shows. Here the automatic vacuum brake failed, 

 and the train ran into a brake-van standing by the stop- 

 blocks, doing considerable damage. Reports of failures 

 of the Midland automatic vacuum may be seen almost 

 weekly in the " Railway Matters " column of the Engineer, 

 and we give, as an example taken at random, one re- 

 ported in the issue for October 17 : — "As it is not very 

 likely to be elsewhere recorded, it may be here mentioned 

 that on the 10th inst. a twelve-coach Midland Scotch ex- 

 press ran clean through Bedford station before it was 

 stopped, in consequence of the failure of the leak-off 

 vacuum brake." Such failures are highly dangerous, and 

 any brake with which they are likely to occur cannot be 

 efficient, and therefore ought not to be trusted to stop trains 

 at any important junction or station, and its use abso- 

 lutely prohibited on approaching a terminal. 



It may be interesting to have a short account of the 

 Westinghouse automatic pressure brake, the worst fault 

 of which, according to its opponents, is its efficiency 

 in stopping trains should anything go wrong with the 



brake apparatus. The motive power of this brake is com- 

 pressed air at a pressure of about 80 lbs. to 100 lbs., com- 

 pressed by an ingeniously constructed steam-pump on the 

 engine, and stored in a main reservoir under the foot- 

 plate ; throughout the train runs a pipe, connected 

 between the vehicles by means of flexible hose pipes and 

 couplings. On each vehicle, including the engine, is 

 placed a small reservoir, a triple valve, and a brake 

 cylinder, with a piston connected by levers and rods to 

 the brake blocks. On the engine is placed the driver's 

 brake-valve for working the brake. The whole system of 

 this brake lies entirely in the construction and action of 

 the triple valve. When the brake is in use, the train- 

 pipe and small reservoirs are charged with compressed 

 air, the air passing through the triple valve in its passage 

 from the train-pipe to the small reservoirs. On the air- 

 pressure being reduced, the triple valve opens a passage 

 between the small reservoir and the brake-cylinder, thus 

 allowing the compressed air stored to expand into the 

 brake- cylinder, forcing out the piston, and applying the 

 brake. To take the brake off, the converse happens : the 

 pressure in the train-pipe is increased, the triple valve 

 closing the passage between the small reservoir and the 

 brake-cylinder, at the same time allowing the compressed 

 air in the brake-cylinder to exhaust into the atmosphere, 

 the small reservoir again being charged with compressed 

 air from the train-pipe. 



The triple valve consists of a small cylinder having a 

 piston connected on the upper side to a small slide-valve 

 working over two ports, arranged one about the other, the 

 lower opening direct to the atmosphere, the upper con- 

 nected by a pipe to the brake-cylinder. The slide-valve 

 works in a small casing connected to the small reservoir ; 

 the triple valve is connected to the train-pipe by a pipe 

 opening into the lower part of the cylinder in which the 

 small piston works. When the piston is at the top of the 

 cylinder it opens a connection between its lower and 

 upper side, thus allowing compressed air to pass round 

 the piston into the casing in which the slide-valve works, 

 then into the small reservoir. When in this position, the 

 slide-valve has closed both ports to the compressed air in 

 the casing, the port leading to the brake-cylinder being 

 open, through the valve, to the lower or exhaust port. 



On charging the train-pipe with compressed air it will 

 be observed that the piston in the triple valve will be 

 forced up, thus filling the small reservoir and triple valve 

 with compressed air, but not the brake-cylinder ; also 

 that the pressure of air on both sides of the piston in the 

 triple valve will be equal ; on reducing the air-pressure in 

 the train-pipe by a few pounds, the piston will naturally be 

 forced down, by the greater pressure on the upper side 

 moving the slide-valve and allowing a quantity of the 

 compressed air in the small reservoir to enter the port 

 leading to the brake-cylinder, and apply the brake. 



The air expanding into the brake-cylinder will cause its 

 pressure to be reduced, and therefore balance the-piston 

 in the triple valve. It is evident therefore that any small 

 reduction of pressure in the train-pipe will cause a cor- 

 responding application of the brake, a reduction of the 

 pressure by 25 lbs. being sufficient to put the brake hard 

 on and skid every wheel. 



The function of the driver's brake-valve is to work the 

 brake-apparatus by varying the pressure of the air in the 

 train-pipe. In the first position of the handle which 

 works the valve, called the charging position, air from 

 the main reservoir is able to go direct to the train-pipe, 

 to charge or release the brake. On moving the handle 

 through an angle of a few degrees into the feed-position, 

 the connection between the main reservoir and the train- 

 pipe is closed, the compressed air having to pass through 

 a pressure-reducing valve on its way to the train-pipe from 

 the main reservoir to make up for any slight leakage 

 which may occur. 



It is important that the pressure of the air in the main 



