Correct use of air tool technology.

Transcription

1 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 1 van 7 Correct use of air tool technology. Air-powered tools are an integral component of the Bosch range of industrial tools. We want to pass this know-how on to you. This guide therefore covers certain essential features of compressed air and its application as a drive medium for air-powered tools. The guide discusses the construction of the motor, maintenance and the system with simple approximate calculations and also discusses aspects of application errors. Air inlet Rotor Sleeve valve Stator Air outlet The driving force: the air motor. The individual tools are designed differently to match the relevant fields of application. The drive motor and its construction, however, always remain the same in principle, apart from different sizes. In the case of hand-held air-powered tools, the sleeve-valve, or vane motor, is the most suitable on account of its high efficiency and compact dimensions. It is driven by the expansion of compressed air and can perform mechanical work. Essentially, the vane motor consists of the cylinder, the rotor which accomodates the vanes in longitudinal slots as well as the sealing plates which seal off the cylinder on both sides, and the bearing mounting. These chambers are mutually sealed as the vanes are pressed against the inside wall of the cylinder during operation owing to their own centrifugal force. The compressed air admitted through the intake passage is forced against the chambers and causes the rotor to rotate. The air intake and air outlet are arranged dependent upon the required direction of rotation. In general, a planetary transmission is connected in front of the motor in order to obtain the correct operating speed. The following characteristic features make the air motor an ideal drive element for an extremely wide range of applications: 38

2 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 2 van 7 The air motor always has a favourable torque behavior for various applications. With increasing load and dropping rotational speed, the torque increases up to a maximum at standstill (Fig. 1) this is utili zed on bolters for instance. The motor can be operated through to stalling. This prevents the possibility of failure as the result of overload. The stalling torque can be regulated steplessly by regulating the pressure of the compressed air admitted (pressure governor). Small overall dimensions and low weight permit fatigue-free and versatile use. The sturdy, uncomplicated design guarantees a long service life and low susceptibility to faults. One further advantage is the insensitivity to environmental influences (dust, moisture etc.). Air-tools offer high operational safety because the driving medium, air, is harmless and, since there is no static build-up, no explosions can be caused (when working in rooms where the danger of explosion exists, follow special instructions). Because the expanding compressed air cools the tool, the machine cannot overheat. The tool can be used easily in wet and damp rooms. Simple maintenance and repair. The air pressure should not be less than 6.3 bar at the tool inlet (flow pressure) so as to guarantee full power at the work spindle. Characteristics of an air motor Starting torque M max Stalling torque Power P P max = n 0 2 M P Speed n 0 Torque M Fig. 1 Characteristics of an air motor For optimal service life: the maintenance unit. Fig. 2 Maintenance unit Despite various measures (drainage systems etc. downstream of the compressor), it is still impossible to avoid the increased cooling of the compressed air with increased line length and the subsequent condensation of water. Scale and rust can also occur, particularly in the case of old lines. If a compressed air filter is installed just upstream of the tool, these components can, however, be eliminated. There should always be a compressed air oiler downstream of the filter in order to admix an oil mist to the compressed air flowing through. This oil is required for lubrication of the air motor, particularly in the case of continuous operation. Maintenance units should be connected as close as possible to the tool. Their size must correspond to the throughput of air at the discharge point. A pressure controller with pressure gauge can be fitted in the maintenance unit between filter and oiler if a specific operating pressure is required or if pressure fluctuations from the line are to be compensated for (Fig. 2). The compressed air must be conditioned with a maintenance unit in order to achieve maximum possible tool service life. Please refer to the operating instructions for air-powered tools for further details. Oil for the maintenance unit or direct lubrication: SAE 20 or SAE 10 engine oil. 39

3 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 3 van 7 The first link in the chain: the compressed air system. Even though Bosch does not produce compressed air systems, the essential construction of such a system should briefly be discussed (please refer to the compressor manufacturer for further details). Compressor: 4 types of compressors are generally in use: Reciprocating compressor Single or two-stage reciprocating compressors are available, depending upon pressure range. For example: single-stage for final pressures up to approx. 10 bar; 2-stage for final pressures up to approx. 17 bar. Rotary compressor Helical compressor Turbo compressor Compressed air reservoir control The compressed air supplied by the compressor is stored in a compressed air reservoir (air receiver) which also serves as a buffering volume for equalizing pressure fluctuations. This covers brief consumption peaks without the operating pressure in the line fluctuating or dropping too far. The air requirements during the consumption peaks should not exceed the supply capacities of the compressor for a long period. The pressure in the reservoir is controlled by the compressor switching off when a maximum pressure is reached (e.g. 12 bar) and switching back on when the pressure drops to a minimum value (e.g. 8 bar). The compressed air reservoir and the supply lines act as storage for the tools during these periods. Idling control This is carried out on medium to large-size reciprocating compressors, generally by opening and closing sleeve-valves or valves. This avoids continually switching the electric motor off and on and the associated high starting current. Start-stop control The start-stop control is carried out on small to medium-size compressor systems via a pressure monitor which switches the electric motor on and off depending on the reservoir pressure. The following rule of thumb applies: V z Q with start-stop control V z 0.4 Q with idling control; where V = receiver volume(m 3 ) Q = delivery rate of the compressor (m 3 /min). Frequently, additional compressed air reservoirs are installed at the end of the line system or upstream of large consumers to compensate for surge loads. Proper dimensioning: the line system. The following simple example shows how to check to what extent the capacity of compressor and compressed air reservoir is loaded when connecting a consumer: Compressor: Delivery rate 1000 l/min (35.3 cfm) Compressed air reservoir: Volume 500 l (17.6 cf) Operating cycle between 12 and 8 bar. The compressor switches off at the final pressure of 12 bar. Until the compressor switches on again at 8 bar, 12 bar 8 bar = 4 bar is available to the consumer and thus 500 x 4 = 2000 l (70.6 cf), i.e. a continuous operating period of 4 minutes with an air consumption of 500 l/min (17.6 cfm). It must be noted that many tools, particularly bolters, are switched on only briefly (approx. 3 seconds). If a percussion bolter, for instance, with an average air consumption of 20 l/s (42.4 cfm) is used 4 times per minute and operated for 3 seconds per bolted connection (i.e. 3 x 4 sec. pure operating time during 1 minute), it actually requires only 20 x 3 x 4 = 240 l (8.5 cf). Thus, 2000/240 = 8.33 min. would pass before the compressor switches on again at 8 bar network pressure. As with the selection of the compressor and the compressed air reservoir, it is also necessary to allow for a possible increase in consumption, for example, through increased production, when setting up the network. In practice, it is generally not possible to avoid cooling of the compressed air in the line. The lines are laid with a slight downgrade of 2 3 % in the flow direction in order to prevent the condensation which occurs from flowing back in the direction of the compressor. Condensate traps can then collect the condensate at the lowest points of the line system. It is conventional to lead the main branch points upward out of the main line (see Fig. 2) in order to also keep the condensate well away from the discharge points. The pipe or hose inner diameter also has a major influence on the performance of the air-powered tools. Lines which are too small increase the flow resistances and result in a corresponding drop in the performance of the machine. When selecting the cross-sections (not less than 3/4 whenever possible on pipelines), the 40

4 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 4 van 7 following influencing parameters must also be taken into consideration: Air flow rate, line pressure, flow velocity, pressure losses. Length of the line. Number and type of line valves and fittings such as bends, elbows, T-pieces, narrow sections, maintenance units and couplings etc. Future increase in air requirements and possible expansion of the system. When determining and checking the line cross-section, it must be taken into consideration that all units are never operated simultaneously. Due regard is paid to this fact by the so called simultaneity factor (Fig. 4). The pressure drop resulting from the resistance in the valves and fittings etc. is allowed for with an allowance of approx. 30 % on top of the actual pipe length. The pressure drop up to the most remote part of the system should not be more than 10 % of the network pressure whenever possible. If pressure losses of 1 bar or more Simultaneity factor 0,95 0,90 0,85 0,80 0,75 0,70 0,65 0, Fig. 4 Simultaneity factor Number of tools occur, the conditions in the line system must be checked. Ring lines are generally used for large line systems since improved supply of the relevant discharge point is guaranteed with increasing load (Fig. 3). Developed by professionals for professionals: common application errors Certain application errors are frequently the cause of unsatisfactory operating results or malfunctions. Common errors are: Tools not correctly selected (machines too weak or too powerful for the application). Inadequate air flow rate and inadequate or non-constant pressure directly upstream of the device. Inadequate cross-section of the supply line. No maintenance units. Dirt, water and a lack of oil results in premature failure of the machine owing to accelerated wear and rusting of the motor. Worn, blunt or unsuitable tools reduce efficiency. Ring line 2-3% downgrade Compressor After-cooler Water separator Air receiver Maintenance unit Lowest point Condensate trap Compressed air reservoir upstream of large actuators Fig. 4 Schematic diagram of a compressed air system 41

5 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 5 van 7 Approximate calculations for line dimensioning. Calculations on the basis of precise equations are too extensive for practical applications. In addtion, individual factors are difficult to record or cannot be recorded at all. Nevertheless, to provide an overview a brief approximate calculation can be carried out using the diagram (Fig. 5) for determining the clear inside pipe diameter. Example: The sum of the air consumption of 6 machines is 36 l/s (76.3 cfm). From Fig. 3, we obtain the simultaneity factor 0.79 for 6 machines. This means 36 x 0.79 = 28.5 l/s (60.4 cfm). This value can be used to dimension the line on the basis of the diagram (Fig. 5). Starting from the air flow rate of approx l/s (60.4 cfm) expanded air, we obtain a clear inside pipe diameter of at least 1. With a theoretical line length of 130 m (actual length % allowance for pressure drops at valves, fittings and bends etc.), we already have a clear inside pipe diameter of 1.5. If further machines are also to be connected to this line in the future, their anticipated air consumption must also be allowed for in our calculations. An existing system can be checked in the same way. Unlike determining the line cross-sections, the compressor size is determined by the operating factor. The operating factor expresses the actual operating time of the unit in percent. On systems to which predominantly screwdrivers are connected, this factor is on the order of approx. 5 to 15%, while for systems with grinders and sanders operated continuously (e.g. casting cleaning shops), a factor on the order of approx. 30 to 70% can be anticipated. However, in order to determine the required compressor sizes as accurately as possible, it is best to check the situation on site and then to estimate the operating factor or to consult a compressor manufacturer. Air quantity (l/s) expanded air 5 6,5 8 12,5 16, , , ,5 133 Pipeline length (m) /2 (13 mm) 3/4 (19 mm) 1 1/4 (32 mm) 1 (25 mm) 1 1/2 (38 mm) 2 (50 mm) 2 1/2 (65 mm) 3 (80 mm) Fig. 5 Line dimensioning 42

6 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 6 van 7 Power through speed control Speed regulation has the following advantages: High grinding/sanding rate Reduced disc usage Time savings Decreased rotating piston wear Less noise development Starting torque M max Stalling torque M P max P The sensitive speed regulator allows a nearly constant working speed, thereby enabling grinding/sanding in the correct area at constant circumferential velocity. With increasing speed, the governour weights (1 and 2) swing outwards, and, as a result, the valve body (3) reduces the inlet cross section. If the speed decreases, the force of the restoring spring (4) Speed Fig. 6 Characteristics with and without speed regulation n with speed regulation witouth speed regulation n 0 n 0 Regulated unregulated Power P Torque M predominates and the cross section becomes larger (Fig. 7). Governor weight Valve body Restoring spring Governor weight Fig. 7 Speed regulation 43

7 CAT_BOSCH_ _EN_bosch_production-tools_catalogus_Air-Tools-for-Trade-and-Service-Workshops_Luchttechniek pagina 7 van 7 Bosch service quality The Bosch CD-ROM Service Information System provides information about Bosch Power Tools over the past 25 years including spare parts lists and exploded drawings, thus saving time and money on spare parts management. Bosch Fax Service offers direct, cost-effective access to important service documents such as spare parts lists, exploded drawings and forms. Information can be accessed 24 hours a day even on Sundays and bank holidays. Bosch Spare Parts Service guarantees in 99% of all cases that the desired spare part is available ex stock, allowing work to continue quickly. Bosch Recycling Service offers environmental protection that anybody can actively take part in. Bosch Industrial Tools, cordless tools and battery packs that are past their service life are taken back at no charge via specialist retailers or directly and sent for recycling (05.05) Printed in Federal Republic of Germany Imprimé en République Fédérale d Allemagne Subject to technical alterations. No liability can be accepted for printing errors. Robert Bosch GmbH Geschäftsbereich Elektrowerkzeuge Marketing, Vertrieb Industriewerkzeuge Postfach D Leinfelden-Echterdingen ProductionTools@de.bosch.com