Arthur J. Weed

Copyright April, 1904 by Arthur J. Weed. All rights reserved.

In designing and constructing a model turbine engine we are confronted with several problems which do not have to be taken into consideration when dealing with a reciprocating engine. The most important of these are: - The high speed; the balancing of motive parts, adequate bearings and a positive oil supply. In the design here given, these details have been carefully worked out, yet the construction is such that it will not be found difficult of accomplishment by any amateur possessing a small lathe fitted with a plain slide rest.

In the turbine engine the principle that "Action and reaction are equal " is most practically demonstrated. The most efficient turbine engine is the one having a comparatively large number of buckets, but to secure this in a model the size of the one here shown, it would be necessary to turn out the motor wheel with a solid rim, like the flywheel cf an engine, and mill out the buckets from the solid metal using an end milling cutter for the purpose. This construction would necessitate the use of special facilities with which few amateurs are equipped.

Before beginning a detailed description of the parts, we will first study the general arrangement of the assembled model. Fig. 1 and 2 show the complete model. From these views it will be seen that the steam inlet is cast as a boss on the side of the casing instead of being a separate piece, for which a hole would need to be drilled diagonally through the casing. The exhaust steam passes out at the bottom of the casing, which facilitates the keeping of the interior free from water. The reduction of speed is accomplished by a worm wheel mounted on the turbine shaft and engaging with a gear on the driving shaft. This permits of a reduction of twenty to one, yet al-ows an increase of speed, if so desired, by substituting a smaller gear on the driving shaft. The side thrust of the jet of steam on the turbine wheel is offset by the thrust of the worm wheel on the gear of the driving shaft.

The shaft of the turbine wheel is made as long as possible, and the outer end supported in an outboard bearing. Each bearing is fitted with an oil ring and reservoir. Where the shaft passes through the casing a stuffing box and gland is introduced. As the pressure at this point is only that of the exhaust steam a piece of felt or candle wicking will be sufficient packing to use. We will first consider the construction of the turbine wheel.

This piece of the model requires'to be very carefully constructed, and when completed the wheel should be in perfect balance. The design requires forty vanes or buckets arranged around the circumference of the wheel. Figs. 3 and 4 show in detail the construction of the wheel. This wheel is made with a solid web between the hub and rim, instead of being spoked; the reason for this being that the entire surface of the wheel may be turned up true to make it run in perfect balance. The shape of the buckets is shown in Fig. 7. The shape of the lower portion of the bucket is spherical with a projection which fies into a slot of the wheel. The forming of these buckets will be the greatest difficulty that the amateur will encounter in the construction of the model. These, however can be procured formed up and trimmed ready to set in the wheel.

To form up the buckets take two pieces of steel 3/8" x 1 i" and about 2" in length. Clamp the two pieces together and drill two i" holes for guide pins near one end, as shown in A, Fig. 19. The opposite ends must be squared up and a center punch mark made on one of the pieces 5-64" from the edge where the two pieces join. Drill a small hole to a depth of |", using a drill about No. 24 size. With the two pieces firmly clamped together drill into the same hole with a i" drill to the same depth and follow this up with a ball end cutting mill as shown in B, Fig. 19. This can be made from a piece of 1/2" steel and the end rounded to a spherical form. The teeth can be filed in and the tool then hardened and tempered. When using this tool it will be necessary to remove it from the work often and clear the teeth of the particles of material cut away from the work. If this is not done the grooves will fill up and the cutter refuse to work. When the depression is cut to the proper depth a piece of 1/2" steel is turned to the same form as the end of the cutter and inserted in one of the pieces of steel as shown in D Fig. 19. This can be fastened in position by a rivet. This forms a punch and die for forming up the buckets. The material of which the buckets are made should be soft brass about No. 26 gauge. This should be cut into pieces considerably larger than the size of the buckets, say one inch square. Place the forming cools with the "die" below and lay one of the pieces of soft brass in position on it. Place the "punch " above it on the guide pins, and holding the three pieces together, transfer them to a large vise and squeeze them together firmly. On relieving the pressure the sheet of brass will be found in the form shown in E, Fig. 19, and only requires to have the flat part trimmed away to make it into a bucket of the proper shape for this model.

After the required number have been formed up, one of them should be very carefully laid out and cut to the size and shape shown in Fig. 7, and this can be used as a gauge with which to mark out the others. Fig. 4, the cross section of the turbine wheel, shows the rim turned out to form the same shape as the bottom of the bucket. This can be somewhat modified as shown in Fig. 5, where the bucket rests on two projections, one on either side of the rim and the centre cut away on a straight line. The latter is the easier construction and keeps the buckets in position equally well.