ENGINEERING RESEARCH INSTITUTE THE UNIVERSITY OF MICHIGAN ANN ARBOR Interim Report PRECISION-CAST ORDNANCE COMPONENTS PROGRESS ON CRANKSHAFT AND CRANKCASE FOR AV-1790 ENGINE FOR M-48 TANK - ~~LrCR. RA. Flinn G. A. Colligan Project 2400 DEPARTMENT OF THE ARMY DETROIT ORDNANCE DISTRICT DETROIT, MICHIGAN CONTRACT NO. DA-20-018-ORD-13832 August 1957

The University of Michigan ~ Engineering Research Institute t L ABSTRACT The present report has been prepared to give the results of the initial development effort on two complex but extremely important parts, namely, the crankshaft and crankcase for the AV-1790 engine (M-48 tank). A discussion devoted entirely to these parts should be helpful at the prestime since funds are being requested to carry on these projects to the final stages of manufacture and testing of the parts. OBJECTIVE The purpose of this investigation is to apply the most advanced precision-casting methods to the production of large ordnance components to obtain substantial reduction in cost or critical materials as well as improvement in design. ii

The University of Michigan ~ Engineering Research Institute CAST CRANKSHAFT With the help of the Continental Aviation and Engineering Corporation, stress analyses of single-throw sections of several precision-cast designs were compared with the present forged product. These measurements have resulted in the design of a lighter replacement which can be cast with a 14$ reduction in stress and with substantial saving in SAE 4340 steel and machining time. In addition to these advantages in processing and design, the survey given in the text indicates that the cost of the cast shaft should be 50% lower than the present forged shaft. It is proposed to proceed with precision-casting crankshafts of this new design using semi-permanent graphite molds with shell-molded cores in some sections. CAST FERROUS CRANKCASE The requirements for components carrying greater loads in advanced tank designhave led to very heavy (4-in.-thick) sections of aluminum alloy in the crankcase. Because of the susceptibility of these heavy sections to porosity and shrinkage, only very low design stresses can be used (3500 psi). An initial model of a lighter, stronger crankcase, which can be made of either cast steel or ductile iron has been designed and a pattern for casting is now ready. It is proposed to cast and proceed with stress analysis of the model section prior to final design and production of a full-size crankcase for service testing. The detailed description of these parts may be given in two separate divisions: the cast steel crankshaft and the cast ferrous crankcase. A. CAST-STEEL CRANKSHAFT, PART NO. 8717036 The production of a precision-cast steel crankshaft offers very great potential savings in machining and material costs. The present forged cranks shaft requires extensive machining and is limited in design because of its method of manufacture. It is estimated that the reduction in machining and material by casting would reduce the final cost of the crankshaft by 50%. 1

The University of Michigan ~ Engineering Research Institute PROCEDURE The general procedure followed in the study of these two components was the same. Initially the design and production personnel of Continental Aviation and Engineering were contacted to assist in the study. A thorough engineering study of possible components was made, based on drawings and field inspection trips. If a part was considered suitable after this, a careful redesign was made to take full advantage of the casting process and flexibility of chemical analysis. At the completion of the phase, new drawings and section models were made for test purposes. To illustrate the importance of the steps of redesign and model construction, Figs. 1 and 2 have been included. These are photographs of the single-throw wooden model produced as a result of the redesign study of the forged crankshaft. This model was split and used as a pattern at The University of Michigan to produce a cast SAE 4340 steel crankshaft test section, Fig. 3. After machining the bearing surfaces to the proper dimensions, the casting was submitted to Continental Aviation and Engineering for stress analysis. Since prior work indicated that torsion was not critical, a comparison of bending stress was made between the forged and the cast shaft. Figure 4 shows the experimental section under testing conditions. A bending moment was applied to the shaft and the resultant strains were measured. Stresscoat was used to determine the location and direction of the principal strains. SR-4 strain gages were applied at these locations to measure the magnitude of the strains. The results of these tests are reported in units of strain. Although the stress is proportional to strain, a Mohr Circle analysis would be required to obtain the absolute magnitude of the stress as the stress is not uniaxialo DISCUSSION OF RESULTS The results of the tests performed by Continental Aviation and Engineering offer an interesting comparison of the forged and'cast sections. 1. Forging —Stresscoat indicated that the maximum principal strain in bending was in the fillet Joining the crankpinand journal. Figure 5 is a photograph showing the strain concentration in the fillet area of the forged shaft. With the applied bending moment of 10,000 in-lb, the maximum strain was 670 p.in./in. The range of strain was from 518-670.in /in. 2~ Casting. —The original design of the casting resulted in a max strain of 650.in./in., a.reduction of only 3% from the forged design. To make a more significant improvement the re-entrant fillet in the cheek was enlarged to.250-in. radius and blended as shown in Fig. 6, The maximum bending strain was reduced to 575 p.ino/in. This modifi^~ ~~~~~~~~~2 ~

The University of Michigan ~ Engineering Research Institute cation results in a 14% reduction of maximum strain as well as a reduction in the range of strain from 152 piin./in. in the forging to 105 ptino/in. in the casting o Figure 7 and Table I give the results of this comparison of forging and casting. TABLE I Desin Position of Bending Load Strain Strain Gage (lb) (tin./in ) Forged 1 100 325 Cast 1 100 290 Forged 2 100 257 Cast 2 100 235 Forged 3 100 330 Cast 3 100 275 Forged 1 200 651 Cast 1 200 575 Forged 2 200 518 Cast 2 200 470 Forged 3 200 668 Cast 3 200 554 Other interesting design features are indicated by Figs. 8 and 9. Figure 8 shows the small increase in strain, 10 p.in./in., as a result of varying the hole size in the cheek of the cast shaft from 1/2- to 7/8-in, diameter. To survey the variation in strain, and to confirm the stresscoat indications, the strain field at the fillet was surveyed as illustrated in Fig. 9 (modified shaft with.250-ino fillet radius). A comparison of the new cast design and the present forged design is presented in Fig. 10, The barrel-shaped lightening holes provide a reduced hole size at the surface and strengthen the shaft in torsion. By locating the lightening hole more centrally in the crankpin, torsional resistance is also gained at the oil hole. The addition of large re-entrant fillets in the cheek results in lower maximum bending strain and a more uniform bending strain distributiono A more detailed drawing of these changes is shown in Fig. 11. Table II contains a comparison of estimated material and matching costs for the present forged and proposed cast design crankshaft, 53

I_ __ PRESENT FORGED DESIGN AV_- 1790 ENGINE NEW CAST DESIGN Fig. 10. Comparison of cast and forged sections of crankshaftc for A'-1790 engine.

The University of Michigan * Engineering Research Institute TABLE II Present Crankshaft Cast 1790 Crankshaft Material SAE 4340 SAE 4340 Weight of stock 418 lb 315 lb Finished weight 250 lb 232 lb Scrap material 160 lb 77 lb Finished cost $500o 00 $250.00 Estimated savings per crankshaft $ 250.00 Estimated production 900 vehicles Total estimated savings $225,000,00 These design advantages, which cannot be obtained in forging, coupled with the savings in machining costs and materials certainly justify the production of full-size cast steel crankshafts for engine evaluation. Based on prior success with the graphite-mold pressure-pouring process, it is proposed to produce the full-size shaft by a combination of graphite mold and ceramic cores. Griffin Wheel Company has submitted a proposal for this investigation. CONCLUSIONS AND RECOMMENDATIONS As a result of the redesign and testing performed on a section model, the following conclusions and recommendations appear justified: lo The new cast-design crankshaft provides a 14% reduction in bending strain and develops a more uniform strain distribution in the most critical areas of the crankshaft 2. The combination of redesign and precision casting by the graphitemold pressure-pouring process will reduce the machining and material cost to 50% of the present forged component. 3. A sub-contract should be negotiated with Griffin Wheel Company for the experimental production of cast crankshafts for ordnance engine evaluation. 5

The University of Michigan * Engineering Research Institute B. FERROUS CAST CRANKCASE Until the present time, sand cast aluminum alloys have been used. exclusively for the 1790 M-48 tank engine and other similar engines (Figs. 12 and. 13). With the increase in design stresses this material is fast becoming inadequate. The heavy sections of this component make it susceptible to shrinkage and gas porosity. Because of this susceptibility, the allowable design stress for this material is extremely low, 5500 psi. In the present crankcase and. to a greater extent in the new diesel engine it would be advantageous to have a material of much greater strength and reliability. A ferrous cast crank case (either steel or ductile iron) offers these possibilities, PROCEDURE Initial redesign work on the crankcase was-undertaken to produce a permanent mold aluminum-alloy case. More recent study of material and the advent of the experimental diesel engine necessitated a change of objective. The high stresses especially developed by diesel service demanded a material of much better mechanical properties. In view of this, two new studies have been pursued. A cast steel crankcase has been designed for production in cast sections for subsequent assembly by weldingo This same design will be used for production of a complete ductile iron crankcase. A test-section model of this new design has been produced at The University of Michigan. This wooden model is suitable for use as a pattern to produce the cast steel sections. A complete test program for evaluating this de'sign and these materials will be performed prior to producing a number of fullsize test crankcases for ordnance evaluation. DISCUSSION OF RESULTS A section of the present design-aluminum crankcase is shown in Fig. 141 A comparison of the present-aluminum and proposed ferrous crankcase can be obtained by reference to Fig. 14, The much greater strength of the ferrous base alloy will permit a drastic reduction in wall thickness. The gain in mechanical properties will be far more than the 3-to-1 density increase from aluminum to iron. Thus the over-all weight of the ferrous case should be approximately the same as that of the present one, but it will be more compact and much stronger. A proposal for the production, welding, and testing of the cast steel crankcase sections has been submitted by Continental Aviation and Engineering. The ductile iron crankcase will be produced at The University of Michigan or by a suitable sub-contractor~. 6

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The University of Michigan ~ Engineering Research Institute CONCLUSIONS AND RECOMMENDATIONS The redesign.and engineering evaluation of the cast aluminum crankcase yields the following conclusions and recommendations:.l The cast alminum crankcase should be replacedbya higher strength cast ferrous base alloy. 2. The new cast ferrous design crankcase should be subjected to a thorough stress analysis prior to producing a full-size crankcase. 3o The stress analysis of the cast steel weidment design will be performed on a five-section full-scale model produced from the single-section wooden model. 4. A sub-contract should be negotiated with Continental Aviation and Engineering for the experimental production and testing of the cast steel crankcase model -and testing of the ductile iron crankcase. Development of a ductile cast iron crankcase would be carried on by The University of Michigan. ^~ 8 ~

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0 -0O Cast steel- modified with.250 radius in cheek _- Forging- present design, P/N 529083 700 -3 2\ A 3 -3 600 St 3~~~~~~~~Uw location a. 500 -2 1 400 300 200 Z ~/ lO00 100 200 BENDING LOAD, POUNDS @ 50.5 IN. Fig. T Comparison of strain versus be nding load. in forged ad cast cra nhaft tresht sectionso

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Fig. 12. Photograph of sand cast aluminum-alloy crankcase for AV-1790 engine. Fig. 13o Photograph of sand cast aluminum-alloy crankcase for AV-1790 engine.