Correct use of air tool technology.

The driving force: the air motor.

Rotor

Sleeve valve

Air outlet

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.

Correct use of air tool technology.

Air inlet

Stator

The individual tools are designed dif-ferently to match the relevant fields of application. The drive motor and its construction, however, always re-main 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 com-pressed air and can perform

mecha-nical work. Essentially, the vane mo-tor consists of the cylinder, the romo-tor which accomodates the vanes in lon-gitudinal slots as well as the sealing plates which seal off the cylinder on both sides, and the bearing moun-ting. 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 in-take and air outlet are arranged de-pendent upon the required direction of rotation. In general, a planetary transmission is connected in front of the motor in order to obtain the cor-rect operating speed. The following characteristic features make the air motor an ideal drive element for an extremely wide range of applica-tions:

Characteristics

of an air motor

Mmax Stalling torque

= n0 2 Speed n0 M P Pmax

Fig. 1 Characteristics of an air motor The air motor always has a

favou-rable 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 re-sult of overload.

The stalling torque can be regulat-ed steplessly by regulating the pressure of the compressed air ad-mitted (pressure governor). Small overall dimensions and low weight permit fatigue-free and ver-satile use.

The sturdy, uncomplicated design guarantees a long service life and low susceptibility to faults. One further advantage is the in-sensitivity to environmental influ-ences (dust, moisture etc.). Air-tools offer high operational safety because the driving

medi-um, air, is harmless and, since there is no static build-up, no ex-plosions can be caused (when working in rooms where the dan-ger of explosion exists, follow spe-cial instructions).

Because the expanding compres-sed 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.

For optimal service life: the maintenance unit.

Fig. 2 Maintenance unit

Despite various measures (drainage systems etc. downstream of the com-pressor), it is still impossible to avo-id the increased cooling of the

com-pressed air with increased line length and the subsequent conden-sation 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 lubri-cation of the air motor, particularly in the case of continuous operation. Maintenance units should be connec-ted 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 re-fer 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.

Even though Bosch does not produce compressed air systems, the essenti-al 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 com-pressors 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

compres-sed air reservoir (air receiver) which also serves as a buffering volume for equalizing pressure fluctuations. This covers brief consumption peaks with-out the operating pressure in the line fluctuating or dropping too far. The air requirements during the consump-tion peaks should not exceed the supply capacities of the compressor for a long period. The pressure in the reservoir is controlled by the com-pressor switching off when a maxi-mum 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 re-servoir and the supply lines act as storage for the tools during these pe-riods.

Idling control

This is carried out on medium to lar-ge-size reciprocating compressors, generally by opening and closing

The first link in the chain: the compressed air system.

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 pres-sure. The following rule of thumb ap-plies:

V z 0.9-1 Q with start-stop control V z 0.4 Q with idling control; where V = receiver volume(m3₎

Q = delivery rate of the compressor (m3_{/min). Frequently, additional}

com-pressed air reservoirs are installed at the end of the line system or

upstream of large consumers to com-pensate for surge loads.

The following simple example shows how to check to what extent the ca-pacity of compressor and compres-sed air reservoir is loaded when con-necting 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 swit-ched on only briefly (approx. 3 sec-onds). If a percussion bolter, for in-stance, with an average air consump-tion of 20 l/s (42.4 cfm) is used 4 ti-mes 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 se-lection of the compressor and the compressed air reservoir, it is also necessary to allow for a possible in-crease in consumption, for example, through increased production, when setting up the network. In practice, it is generally not possible to avoid coo-ling of the compressed air in the line.

The lines are laid with a slight down-grade 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 up-ward 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 perfor-mance of the air-powered tools. Li-nes which are too small increase the flow resistances and result in a cor-responding drop in the performance of the machine. When selecting the cross-sections (not less than 3/4˝ whenever possible on pipelines), the

Proper dimensioning: the line system.

Ring line 2-3 % downgrade Air receiver After-cooler Water separator Lowest point Condensate trap Compressed air reservoir upstream of large actuators Compressor Maintenance unit following influencing parameters

must also be taken into considera-tion:

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, mainte-nance units and couplings etc. Future increase in air requirements and possible expansion of the sys-tem.

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 resi-stance in the valves and fittings etc. is allowed for with an allowance of approx. 30 % on top of the actual pi-pe 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

occur, the conditions in the line sys-tem 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).

Fig. 4 Simultaneity factor

0,95 0,90 0,85 0,80 0,75 0,70 0,65 0,60 1 5 10 15 Simultaneity fact or Number of tools

Fig. 4 Schematic diagram of a compressed air system

Certain application errors are frequently the cause of unsatis-factory operating results or mal-functions. 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 ma-chine owing to accelerated wear and rusting of the motor. Worn, blunt or unsuitable tools reduce efficiency.

Developed by

professio-nals for professioprofessio-nals:

common application errors

5). Starting from the air flow rate of approx. 28.5 l/s (60.4 cfm) expan-ded air, we obtain a clear inside pipe diameter of at least 1˝. With a theore-tical line length of 130 m (actual length 100 + 30 % allowance for pres-sure drops at valves, fittings and bends etc.), we already have a clear inside pipe diameter of 1.5˝. If fur-ther machines are also to be connec-ted to this line in the future, their anticipated air consumption must al-so 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 expres-ses the actual operating time of the

unit in percent. On systems to which predominantly screwdrivers are con-nected, 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 ap-prox. 30 to 70 % can be anticipated. However, in order to determine the required compressor sizes as accura-tely as possible, it is best to check the situation on site and then to esti-mate the operating factor or to con-sult a compressor manufacturer.

Fig. 5 Line dimensioning

2 1/2’’ (65 mm) 1/2’’ (13 mm) _3/4’’ (19 mm) 1’’ (25 mm) 1 1/4’’ (32 mm) 1 1/2’’ (38 mm) 2’’ (50 mm) 3’’ (80 mm) Air quantity (l/s) expanded air 10 20 30 40 50 75 100 150 200 250 300 350 400 450 500 5 6,5 12,5 16,5 25 33 41,5 50 58 66,5 75 83 100 110,5 133 Pipeline length (m)

Approximate calculations for line dimensioning.

equations are too extensive for prac-tical applications. In addtion, indivi-dual factors are difficult to record or cannot be recorded at all. Neverthe-less, to provide an overview a brief approximate calculation can be car-ried 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 fac-tor 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.

Fig. 6 Characteristics with and without speed regulation S tar ting t or q

ue MmaxStalling torque

Speed

n n0_Regulated Pmax

n0 unregulated

with speed regulation witouth speed regulation M P Power P Torque M

Speed regulation has the following advantages:

High grinding/sanding rate Reduced disc usage Time savings

Decreased rotating piston wear Less noise development

The sensitive speed regulator allows a nearly constant working speed, thereby enabling grinding/sanding in the correct area at constant circumfe-rential 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)

Fig. 7 Speed regulation

Restoring spring

Governor weight

Valve body

Governor weight

Power through speed control

predominates and the cross section becomes larger (Fig. 7).

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