Control
A number of different methods of control are available within the category of pneumatic screwdrivers:
- Lever Start- A lever is pulled by the hand to open the valve, allowing the air into the motor which starts the screwdriver.
- Trigger Start- Generally found on pistol grip pneumatic screwdrivers, the trigger operates on the same principal as the lever start except the tool has a trigger style button in place of the lever.
- Auto Start or Push to Start- When the output is pushed onto the fastener head, this opens the tool’s valve and allows air to the motor which starts the screwdriver.
- Remote Start- Generally used in fixtured operations, a pneumatic circuit is used to control the screwdriver operation.
Motor
Pneumatic screwdrivers use a vane motor. This type of motor is very high speed, but low torque. In a vane motor, air enters the inlet at 90 PSI. This air exerts pressure on the faces of the exposed blades. The pressure can act over a greater area (PSI) on one of the blades. As the other side of this blade is at a lower pressure, a differential is created causing the rotor to turn.
Due to the nature of a vane motor, a number of factors can affect it’s performance:
- Airflow- The greater the pressure of the air entering the motor, the greater the pressure differential created. This causes the motor to rotate quicker. If there is too much pressure entering the motor, the motor will turn too quickly wearing the bearing and blades out (the blade tip speed on the cylinder is greatly increased).
- Exhaust- If the exhaust becomes blocked, this will prevent the air from escaping, causing back pressure and reducing the differential accross the blade. The motor will then rotate slower, with less power.
- Blade Condition- If the blades in the motor are worn out or in poor condition, the pressure difference between the blade chambers cannot be maintained. Regular lubrication is necessary to keep the blades in good condition.
- Air Condition- If the air supply is dirty, this will prevent the blades from sliding in & out of the rotor and keeping contact with the cylinder.
- Overall Motor Condition- The motor components (bearings, blades, rotor, and cylinder) should be kept clean and in good condition to allow a free spinning motor.
What determines the strategy and tool type?
One of the first things to consider when selecting a new screwdriver or nutrunner for a particular application is the type of clutch that is best suited for the application. The following information describes the functional arrangement of each type of clutch and the types of applications for which each is best suited.
Direct Drive Clutches Direct connection between the motor and the driving bit. Fastener is driven until the motor stalls. This is the simplest and least expensive of all drives. Low maintanence, diaphram-type pressure regulators can be used to adjust the stall torque. Reducing the stall torque in this type of tool also reduces the operating speed. Motor wear, fluctuations in pressure, and faulty lubrication are all factors that will cause variation in delivered torque. “Kick” can also be fatiguing to operators.
Positive Clutches Motor and driving bit are connected through one clutch, the jaws of which have sloping faces and are normally held apart by a light spring. This clutch provides a stationary bit while locating on a fastener even when the motor is running. When applied to a fastener, the operator’s axial pressure engages the jaws. Direct drive results until torque buildup is sufficient to cause jaws to cam out of engagement against operator pressure. Jaws ratchet, causing further torque buildup until the operator stops the tool or removes it from the fastener. With a positive clutch, the operator’s technique can cause considerable variations in delivered torque. Frequently used to drive screws into wood and similar materials where torque requirements may vary due to knots, soft spots, etc. Not recommended where excessive axial pressure on fastener may damage assembly. Axial impact action of ratcheting jaws may also cause damage to some assemblies.
Adjustable Ratcheting Clutches Motor and driving bit are connected through two clutches. One has 90 degree jaws held apart by a light spring. This clutch provides a stationary bit while locating on a fastener even when the motor is running. When applied to a fastener, the operator’s axial pressure engages the jaws. The second clutch consists of two jaws with sloping faces held together by the compressive force of a heavy spring. This force can be adjusted by means of a nut. When torque is sufficient to cause the jaws of the second clutch to cam apart against the force of the spring, jaws disengage and re-engage repeatedly. This ratcheting action causes further torque buildup until the operator stops tool or removes it from the fastener. A good general purpose clutch for applications where close torque control is not required. Adjustment should be set so desired torque is achieved shortly after jaws begin to ratchet. Sound and feel of ratcheting signals the operator to stop or remove the tool from the fastener. As the operator’s reflexes slow, due to fatigue or distraction, torque can become excessive. Noise and vibration from ratcheting torque can present problems.
“One Shot” Clutches Motor and driving bit are connected through two clutches. One has 90 degree jaws held apart by a light spring. This clutch provides a stationary bit while locating on a fastener even when the motor is running. When applied to a fastener, the operator’s axial pressure engages the jaws. The second clutch consists of a pair of jaws with precision machined pockets. Hardened balls rest in the pockets and are clamped between the jaws by a heavy spring. Torque setting depends upon compressive force in the spring, which is adjustable. When desired torque is reached the balls tend to roll out of thier pockets, forcing the jaws apart. This action separates the 90 degree jaws and completely disconnects the motor from the bit. The motor free-wheels and no further torque is applied to the fastener. Excellent for practically all applications, especially torque-critical jobs such as driving fasteners into soft materials or clamping brittle materials. Instantaneous clutch action is quiet and vibrationless. Clutch maintanence is minimal due to the absence of ratcheting. Not recommended where torque requirements from fastener to fastener are not uniform (due to mis-alignment of parts, etc.) or for rare applications where torque peaks higher than final torque is encountered during self-tapping or thread forming.
Capacity In determining the size of the tool required, consideration must be given not only to the size of the screw to be driven and the final torque required, but also to the nature of the job. The maximum capacity of a screwdriver or nutrunner varies for each type of application. The torque imparted to a screw or nut can come from three sources: (1) From air pressure against the rotor blades, (2) From flywheel effect of the motor and other rotating parts, and (3) from clutch jaw impact of ratcheting clutches. Heavy turining resistance on the run-down reduces speed and consequently reduces torque available from flywheel effect and jaw impact. Resilient assemblies absorb both flywheel energy and jaw impact and thereby reduce torque produced by these two factors.
The maximum torque a given tool can impart to a screw or nut is greatest when driving “free-running–sudden stop” screws or nuts used to fasten non-yeilding materials. Here the resistance to turning is low throughout the run-down and the tool runs at close to it’s free speed. Maximum possible torque is imparted from each of the three sources mentioned above when the screw or nut seats against a solid stop.
Normally, a particular tool will deliver lower torques on a “soft pull up” job than any of the other types of jobs since the heavy resistance to turning reduces the amount of torque obtained from the flywheel effect. On other types of jobs illustrated at the right, a tool will generally deliver slightly more torque than the maximum “soft pull up” rating.
For example, a “soft pull up” condition would exist in the case of a sheet metal screw if the screw is head seated before the hole in the sheet was expanded to the maximum root diameter of the screw, since resistance to turning would remain high throughout the entire run-down. If, however, the screw were longer and considerable run-down occurred after the hole in the sheet was expanded to maximum diameter, the tool could be expected to pick up speed due to the decrease in resistance and the energy available from the flywheel effect and impacting would increase. Thus the job would change from a nominal “soft pull up” condition to something in between that condition and “free running–sudden stop”.
Impacting is not a factor with “One Shot” and direct drive tools. Flywheel effect causes a noticable variation in torque delivered by direct drive tools when job conditions are not uniform. This variation is less pronounced in “One Shot” tools.
Reversibility Non-reversible models recommended for all normal assembly work. Reversible models are suggested for all disassembly work and for assembly where screws must be frequently removed to cross threads or defective threads.
Speeds The “free speed” of a screwdriver is the rpm of the drive spindle with no load, 90 lbs. of air pressure, and with the speed regulator wide open. Different “free speeds” are obtained by using different gear ratios between the motor and the drive spindle. The actual speed of any tool can be reduced if desired by using the speed regulator to adjust the rate of air flow to the motor.
With Direct Drive select a tool with a “free speed” whose maximum stall torque is equal to, or greater than, the torque required by the heaviest screw to be driven. Then use the speed regulator to reduce the speed and power the tool to give exact torque required.
With Adjustable Ratcheting Clutch high “free speed” usually gives greater clutch jaw impacting effect, and will drive free-running and light pull-up screws tighter. Reduce speed with the speed regulator when less impacting effect is desired. Tools with slow “free-speed” can deliver greater torque on long, heavy pull-up jobs.
With “One-Shot” Clutches, the degree of torque accuracy generally increases at lower speeds. The lowest speed “One-Shot” tools are practically insensitive to variations in job conditions. A certain amount of torque variation should be expected from higher speed tools but thier accuracy will still be adequate for most production jobs.
Such variations will be most pronounded on jobs approaching “free running–sudden stop” conditions. When the allowable torque range specified for a fastener is extremely narrow, the lowest speed tool having sufficient torque capacity is recommended.
Handles In most instances a straight handle is used for driving verticle screws, and an offset or side handle for driving horizontal screws. However, the two governing factors are the position of the operator’s forearm and the position of the screw. If the screw and the operator’s forearm lie parallel to each other, use an offset or side handle. If they are perpendicular, use a straight handle.
Straight handle with overhead suspension balancer is a popular arrangement for bench assembly work where parts can be arranged so that screws can be installed vertically. Offset or side handles are better for resisting torque reaction and recommended for heavier work, particularly for slow speed direct drive tools.
Final Notes
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If the torque specified is high, a straight case tool should not be used unless the torque reaction is reduced to the operator by mounting in a bench stand or utilizing a side handle.
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The torque tolerance specified should determine whether or not standard tooling is acceptable. If the tolerance is too tight, it may not be possible to use a
handheld pneumatic screwdriver.
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Wood screws, self-tapping screws, and machine screws all require different types of clutches for optimum performance.
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If a complex procedure is required such as tighening, loosening, then re-tightening, special consideration must be given in tooling selection.
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If the joint type varies, a number of different tool settings may be necessary to give the same torque.
