Metal Construction
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Metal Construction |
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For the year 2006 again a radio controlled large scale truck model was planned - with a motor
bonnet mighty as possible because the intended on-board camera should be able to provide the
outlook of an oldtimer-truck driver.
While searching an adequate original at first the legendary Krupp Titan was taken into consideration. But because of his strong rounded motor bonnet the reproduction with metal kit parts seemed to be relatively difficult. So at last the surely equally legendary Buessing 8000 from the fifties and sixties was chosen.
At a revision in the year 1952 with the same stroke volume the power was increased to 180hp. Also an air pressure aided servo steering now was available, recognizable at the enlarged box on the left side of the motor bonnet. For the different applications moreover different driverīs cabs were offered. So for the long-distance traffic a cab with appended sleeping berth was available which extended into the loading space (swallowīs nest). The production of the Buessing 8000 finished in 1957, but in details modified it was built until the end of the fifties.
In consideration of already existing wheels with a diameter of 160mm followed a scale of approx. 1:7,4. The transfer from the original onto the model took place by means of pictures from the internet as well as diverse plans provided by helpful model builders. Already like at the 1½-Deck-Bus 5mm-square paper was proved to be very practical. From the outset the construction again was arranged on a radio controlled operation, whereby the modelīs mass of approx. 20kg demands respectable forces resp. torques from the driving components. The cogwheels and the shafts of the drive thus do not consist of metal kit material, because also with regard to a possible trailer operation and the thereby expected total mass of 30-40kg brass cogwheels modul 38dp and 4mm-shafts seem to be unsuitable. Instead of that steel cogwheels modul 1 from the Maedler company and 6mm-steel shafts are used, which in the sence of a long durability are mounted in a total of 14 miniature roller bearings. However to use as much as possible metal kit parts all gear reductions are designed to meet the 1/2-inch-grid. Excepting some more industrial available small parts and screws the used metal kit material (nearly) solely was purchased as new merchandise from Metallus. The construction of the frame occured by means of the existing plans. It consists of moulded stringers, which are coupled with traverses at several points. For approximation of the different cross-sections mainly angle girders and L-section angle girders are used.
The adaption of the front wheels to the 4mm-shafts is realized by 65mm large flanged wheels, which exactly fit into the rims and allow a proper centering. With a view to a long durability also the front wheels are provided each with 2 roller bearings.
For transmission of the torque from the drive shafts to the drive wheels the rims are prepared with a 19mm hexagonal recess and a 12mm hole. The adaption between rim and shaft therefore is realized by means of a prepared hexagon cap screw M12x50, which is inserted onto the shaft and clamped by a binding screw.
Because of the lined tyres the steering torque is reduced to approx. 75Ncm, which still is a large value. Therefore a quarter-scale-servo is used with a torque of 180Ncm at a supply voltage of 6V. The high torque of the servo of course requires an adequate high current, and referring to this some precautions are to attend: Unfortunately this servo is only provided with the usual 3-pin line and hence misleads to the connection at a receiver resp. speed controller with BEC-system (Batterie Eliminating Circuit). Since during load the current can increase 2A the BEC-system can be overloaded and possibly destroyed after a short time. On the other hand it is not allowed to connect the servo next to the 7-cell accumulator (8,4V) because it is only specified up to 6V. For the operation of such efficient servos therefore a separate 6V source with sufficient power is necessarry. In the simpliest case this can be realized by means of an overload and short circuit protected voltage regulator, which is supplied from the accumulator. For protection against overload the steering mechanism contains a spring element (servo-saver) which in case of need absorbes the force of the servo.
This vehicle as first model is equipped with a conventional drive system consisting of a front engine with primary gear reduction, displaceable intermediate shaft, change-speed gear, cardan shaft and the rear axle with angle drive, gear reduction and differential.
The change-speed gear is situated behind the driverīs cab and connected to the drive unit via intermediate shaft leading under the driverīs cab. With regard to a possible trailor operation first a gear with 3 reductions was intended, which can be changed remote controlled.
By means of a cardan shaft the torque is transmitted to the drive axle. Because of the spring suspension of the axle normally the cardan shaft requires a length compensation. In the present case the length compensation is realized by using cogwheels with sufficient width of tooth and sufficient clearance in the change-speed gear. Through that in all situations enough gearing is warranted.
The brake shoes consist of appropriate bended and stiffened double angle strips with a glued on rubber brake covering. Because of constructive reasons the brake shoes had to be mounted in this way, that the brake force only in one direction is reinforced due to the self-locking (here backwards). As brake drums the rims herself are used, which with a diameter of 65mm contain all components without difficulty. The operation of the brake also occurs with a quarter-scale-servo, whereby the brake forces can be adjusted separate for each wheel.
As further feature the motor-sound generator is to mention, which mainly consists of 2 low-cost modules from CONRAD and a selfmade periphery.
With the exception of the sound generator the complete electronic equipment is placed in a special mounting above the drive unit. To protect this parts against exceeding contamination with oil the cogwheels of the drive unit are covered with metal sheets.
How already mentioned also the axles of this model have a spring suspension. For this after completion the axle loads were measured with a spring balance and under consideration of the spring deflection (approx. 30mm) and the number of springs the required spring rates were calculated. All springs could be made from one blank and differ only in the number of effective turns. Because the springing of the original does not consist of compression springs but flat springs, the springs of the model are located in this way that they are widely masked by the wheels. For the improvement of the appearance in the well visible areas of the rear axle flat spring dummies without functionality are mounted.
At the beginning of 2006 the model started running, what in view of the weight and the resulting forces resp. torques didnīt proceed without problems. So the powerful steering servo firstly was operated at the BEC-system of the speed controller resulting into a destruction of the concerned voltage regulator. This damage however could be repaired by replacing with a similar regulator. While operating the steering servo at the onto 7V reduced accumulator voltage a part of the servo circuit burned off. Also this damage fortunately could be repaired. Brake tests with full weight showed, that the brake shoes deformed because of the high forces. Also one cardan joint collapsed because of the high torque. But after repair resp. elimination of this faults the model works without further problems until now. Every quarter scale servo now is operated from a powerful voltage regulator, and the brake shoes were stiffened additionally. Also the damaged cardan joint was repairable, but for a possible replacement a stronger type was supplied. In addition the change-speed gear is to mention, which can be switched very smooth at the running model although the missing clutch and synchronization. While the noiseless change up to the high gear is very easy, the noiseless change down to the low gear with "intermediate throttle application" requires some exercise. The operating time with a complete loaded accumulator is approx. 45 minutes. This corresponds to an average current of 3,2A and with a voltage of 8,4V an average electrical power of just under 27W. Regarding the low efficiency of this motors of approx. 55% and with neglect of the servo power follows an average mechanical power of just under 15W. Comparatively to the 1½-Deck-Bus with approx. 11W mechanical power this seems to be realistic, because this model has 4kg more mass.
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