Applications on pneumatic:tool classification and performance.

TOOL CLASSIFICATION AND PERFORMANCE

Tools operated by compressed air can adopt a variety of different forms as in Table I. They can be broadly classified according to the method of operation - percussive, rotary or combination (a combination tool is one in which percussion and rotation is provided pneumatically in the same tool, as in some rock drills and impact wrenches). A further broad classification is according to the place of use- the usual distinctions being industrial (or workshop) tools for factory operations, contractors tools for use on construction sites, and mining tools for quarry and underground use. Although the general principles of design are the same, the practical realisation of those principles can be very different according to the needs of the user.

Industrial tools form part of the total factory environment and as such they may be automatically operated and controlled. In a factory, tools which rely for their motive power on compressed air may nevertheless for convenience be controlled electrically.

In other situations, for example in mining and quarrying operations, the percussion action of a tool may be generated pneumatically whilst the rotary action may be hydraulic. The optimum configuration uses the ideal properties of each medium.

Performance information

Table 2 provides data for typical air consumption of tools. There is much variation between manufacturers. Efficiency is always improving, so it is wise to obtain up-to-date values direct from manufacturers. Air consumption figures are always available but other performance figures for tools are not so easily obtained.

Torque values for air motors, screwdrivers and nutrunners should be supplied and are easily checked by a conventional dynamometer, but the energy output of percussive tools is rarely provided, and for good reason. It is only in recent years that reliable techniques for the measurement of blow energy in percussive tools have become available, these are now embodied in BS 5344.

The technique described in that standard requires the use of strain gauges attached to the drill or chisel bit of the tool. When used with the appropriate analysis equipment, the energy content of each shock wave can be determined. Before this standard was

Drill A rotary tool driving an output spindle generally through a gear box. The output spindle is normally fitted with a chuck or Morse taper or other socket, making the tool suitable for drilling, reaming, tube expanding and for boring metal, wood and other materials.

A percussive tool of heavy construction with rotating chuck for drilling holes in rock, used with a suitable support.

A drifter, slide mounted on a carriage or cradle. The carriage can be mounted on a wagon drill, crawler, hydraulic boom, etc. The feed of the drifter along the carriage can be by chain, screw or cylinder.

A hammer which is placed at the end of a drill rod and entirely enters the hole as it is being drilled. Rotation is effected by an independent motor turning the drill rod to which the hammer is attached.

A heavy percussive machine mounted on a tractor for breaking stone, concrete, etc.

developed, the performance of a percussive tool was assessed by such unreliable methods as measurement of the permanent deformation of a soft pellet when hammered by the tool bit or by the height of the pressure pulse when the tool output is absorbed by an hydraulic buffer. These methods are now only used for comparison between similar tools, and should not be quoted as an absolute measure of tool performance.

Even when the stress energy content of the shock wave is known, it will not necessarily be helpful to the user of the tool, who is concerned primarily with the amount of material that can be removed or drilled. Another way of defining the power output of a percussive tool is to quote, for example, the drilling speed in a specified kind of rock, or the amount of concrete that can be broken by a concrete breaker in a given time, or the amount of steel that a chipping hammer can remove. None of these is particularly easy to measure under controlled conditions. The prospective purchaser should be aware of information pre­ sented in such a way when trying to compare different tools. There is really no alternative to actually trying out a tool in the circumstances of one's own application.

As a general statement, a tool made by a reputable manufacturer will have a power output proportional to the its consumption, and that is probably the best guide that can be given. The weight of the tool is another useful indication of the power, but it should not be assumed that more actual work will be done by the heavier tool.

A further warning should be given when assessing raw performance data. At first sight it might be thought desirable to choose the tool with the highest output, paying no regard to the ease of use. All tools require the operator to apply a feed force and to sustain the vibration present at the handle. Usually a tool with the highest performance is the hardest to handle, but some tools are proportionally worse than others in this respect. Many tools are coming on the market which have been designed for ease of use; feed force, vibration levels and noise have been reduced, although sometimes at the expense of bulk and manoeuvrability.

Performance related to air pressure

Table 2 quotes air consumption for a variety of tools at 6 bar gauge pressure. Most manufacturers use this as a standard at which to measure performance, in spite of 6.3 bar being the recommended ISO standard; most compressed air systems should be capable of supplying this pressure as a minimum. The performance at other pressures is not necessarily proportional to the pressure. Theoretically the power output of percussive tools varies according to (pressure)1.5, but tools are designed to work ideally over a limited range of pressures, and they may not work anything like as well at a different pressure.

Some tools, particularly rock drills and down-the-hole machines for quarrying, work at much higher pressures (up to 20 bar). The reason for these high pressures is the need to maximise the drilling speed within the restricted space limitations of the tool. The overall efficiency of power conversion in high pressure tools may be lower than for standard pressure tools, but efficiency is less important than high performance in these applications.