Technical Solution of the Drive System for Mixing Dough Machine

. In the conditions of the global economy, food industry products have a good share in international trade. It is primarily ready-made food products that are ready for use. The basic problem that needs to be solved is the safety and quality of food products. Machine “Planetary mixer” is used for mixing and preparing dough. From the aspect of optimal fulfilment of the working function of the preparing dough “Planetary mixer” has to fulfil the following conditions: - the complete space in which the dough is located must be treated equally with the mixer; - noise and vibration reduction should be ensured and appropriate precision positioning of the machinery's executive bodies; - a compact construction of the drive system is required with the optimum utilization of the available resources. In the paper is presented technical design solution of the drive system for machine based on the demands of regional medium size enterprise and therefore it is limited to the requirements of the investor. The final solution was achieved by using the modern methods of product development.


Introduction
Machines in the food industry have a great impact on the quality and safety of food. In this sense, the production of these machines must meet the criteria for the production of machines and equipment that include criteria for design solutions, criteria for applied materials and criteria for technical design standards. Special attention in the design solutions addresses the adequacy of cleaning and sanitation facilities and working conditions in extreme conditions (high or low temperature), etc.
Machine "Planetary mixer" is used for mixing and preparing dough. Planetary mixer PM 100 has a dough bowl with diameter of 500 mm, a volume of 100 liters, so that the minimum dough capacity can be 5 kg and maximum 65 kg. The power of the electric motors is P = 4 kW, with the speed n M = 1400 min -1 . Speed control is performed via the frequency regulator. The dimensions of the machine are 700x1500x1100 mm and the total mass of the machine is 550 kg. Depending on the customer's needs, a number of execution tools -mixers (wire, spatula, hook) are delivered to the machine. The basis of the machine is a massive steel construction, which ensures stability in operation. In accordance with standards, the parts of the machine and the tools that are coming in contact with food are made of stainless steel.

Existing solution for machine power
Machine "Planetary mixer" consists of the following assemblies and components (Fig. 1 The planetary gearbox (Fig. 2) is a special construction, with only one planetary gear and no built-in central gear. On the opposite side there is a mass for balancing the centrifugal force. The torque is applied to the planetary gear carrier, which is connected with a fixed toothed wreath. At the same time, the planetary gear performs a complex movement around its own axis and the axis of the gearbox. For the shaft of the planetary gear, an executive mixing tool is directly connected. Thus the mixing tool of the test performs a complex movement turning around its own axis and around the axis of a crowned gear and in this way mix the dough over the entire volume of the dough bowl.
For the given data in Table 1, the calculation of the geometry and carrying capacity of the planetary gear according to DIN ISO 10823: 2006 was performed - Table 2.
The toothed wreck of the planetary gearbox is made of plastic Polyoxymethylen, which in technical literature and market is kept as an abbreviation of POM. It is a semi-crystalline thermoplastic mass produced by polymerization. It has a relatively low friction coefficient (μ zm = 0.04 -0.1), which allows it to have low slip resistance and consequently reduced wear. It has a satisfactory stiffness, and its price is relativly low. However, these characteristics of POM are only available at low and medium operating temperatures.
The calculation of the capacity of the planetary gear in Table 2 shows that load carrying capacity is not satisfactory. This problem is related to the load carrying capacity of a crowned gear. The toothed wreath is in conjunction with the planetary gears made of heattreatable steel 34CrMo4. The operating stress on the toothed flank is σ Hw = 80.83 N/mm 2 , while the critical value is σ HG = 67.5 N/mm 2 , so that the safety level S Hw = 0.84 < S Hmin = 1. The operating stress voltage of the foot of toothed gear is σ F = 115.65 N/mm 2 , while the critical stress is σ FG = 77.2 N/mm 2 . The safety level of the foot is therefore less than the permitted S F = 0.67 < S Fmin = 1.4, or it can withstand the maximum power P OVmax = 1.91 kW. Because of this, in the exploitation conditions, the toothed gear of the toothed cone of the planetary gear was damaged.

Selection of variants of technical design solution
Since the standard gear and chain drive system of the propulsion system of the machine, the planetary mixer has a satisfactory load capacity, there will be no changes in the improvement of the technical design solution in that part. It is necessary to make improvements to the planet drive of the drive part. The reconstruction should be carried out so that the costs are minimal and that the machine's operational function is fully met. So the new solution of the planetary gearbox should be incorporated into the existing available space, which means that the axial distance of the gear unit remains the same a = 115 mm. In this sense, the carrying capacity of 4 possible variants of the gear unit was carried out, ie 2 variants with a module m n = 2.  Table 3. The critical element of the gearbox from the aspect of load carrying capacity is the planetary gear made of plastic material.  The further analysis carried out shows that the best variant is 3, because it can withstand the maximum power of P max = 3.78 kW. The value indicated is slightly less than the required power P = 4 kW. However, it should be known that the calculation was carried out with an input power of P = 4 kW, while the actual power is less due to losses in the gear and chain. If we take the gear ratio of the gearing 0.95 and the chain 0.985, then the power transmitted by the planetary gear P = 0.95x0.985x4 = 3.74kW is obtained. Accordingly, the load capacity of this variant fully meets the required exploitation conditions.

Improved solution for machine power
The selected variant of the planetary gearbox has been elaborated in detail. The planetary gearbox gets power over the input shaft and sprocket L 2 (Fig. 2). The power is transmitted to the planetary gear carrier, which is a connected with the drive input shaft of the gear unit with pin (Fig.3). The planetary gear is coupled with a corrugated gearing, which is immovable and firmly connected to the body of the machine planetary mixer.
Planetary gear carrier has a special technical design, thus executing a number of functions. The most important function is the transfer of power from the input shaft to the planetary gear, whereby it must provide adequate clutching of the planetary gear and the corrugated gearing. Additionally, the planetary gear bearing also provides the mounting of planetary gear, with 2 bearings -6208-2RS1. On the opposite side of the planetary gear, mass is set to balance the centrifugal force.
For the outlet end of the planetary gear shaft, the mixing tool is assembled. The connection is made by a threaded connection, so it enables quick replacement of the toolmixing dough.
The load carrying capacity of plastic gears is affected by deformations of the teeth connected primarily to bending the tooth itself. The calculation of plastic gears is made according to VDI 2736: 2013. According to the calculation, the bending of the toothed gearing is f a = 347 μm, while the allowed value of the f azul = 350 μm, so that the level of safety is S del = 1.007. Therefore, the load carrying capacity of the planetary gearbox fully meets the necessary exploitation conditions. An important parameter of the drive is that the gearbox has adequate precision, which enables adequate precision of positioning, noise reduction and vibration and energy efficiency. This is taken into account here through the choice of quality of production and tolerance. For real conditions, the quality of planetary gear surfaces is 6, the quality of the plastic gearing 7 as well as the tolerance of the tooth thickness DIN 3967 cd 25 were adopted, so that the calculated values of the normal lateral gap j nw = 176 μm, or the circular lateral gap j tw = 187 μm were obtained. This ensures sufficient precision of positioning, noise reduction and vibration as well as energy efficiency. a b For optimum fulfillment of the working function of the machine "Planetary mixer" PM 100 it is necessary to have the entire space in which it is food located to be treated by the executive body of the mixer evenly, which means that the mixer in the space occupies a different position at each turn. In this way, the homogeneous structure of the food in preparation phase is obtained and improves the quality of the food products. This condition is ensured by an adequate choice of the number of teeth of the planetary gear and the plastic gearing. The number of teeth is thus chosen to come in contact with each other at all times, with different teeth of coupled gears (Fig. 4a). As a result, the dough mixing tool pathways always pass through the various points within the dough bowl working area. The trajectory of the movement of one operating edge of the tool in the working area of the dough bowl is shown in Fig. 4b. As the tool has multiple operating edges, it is ensured that the entire space in which food in preparation phase is treated is evenly.

Conclusion
Machine "Planetary mixer" is used for mixing and preparing dough. From the aspect of fulfilling the working function of the mixer for dough "Planetary mixer", the role of noise, precision of movement and reliability in operation is essential.
Innovative technical design solution of the machine, shown in the paper, fulfills the conditions for optimum fulfillment of the operating function of the machine.
1. The complete space in which the food is located is uniformly treated by the executing tool of the mixer, in that the mixer in the space occupies a different position at each rotation.
2. By selecting appropriate parameters related to tolerances and making planetary gear elements, adequate accuracy of positioning, noise reduction and vibration and energy efficiency are ensured.
3. The adopted planetary gear solution is optimal from the aspect of the available resource of its vital elements. The level of safety of the toothed pair of planetary gear units (foot, flanks and deformation of the teeth) are within the allowed limits with the maximum use of the available load carrying capacity.