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Hot Air Station Diaphragm Pump + others - description

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Fraser:
 I recently purchased a new 'customer return' Xytronic LF852D Hot Air Station from Rapid Electronics. It was cheap and I thought the odds of repairing it were good  ;)

The unit arrived and I quickly diagnosed a problem with the pump that had caused it to be returned to Rapid. I own several hot air stations but have never dismantled the pump so its contents were unknown to me. For those in a similar position I provide a text description of the pump design. Sorry I didn't take pictures but there isn't much to see anyway.

The LF852D is the latest 852D clone and is a completely different shape to previous models and it uses a different (better?) pump. The heater is rated at 600W instead of the usual 250W. The LF in the name means 'Lead Free' as the unit is designed for that use.

Once the cover has been removed (a quality aluminium extrusion) the Pump is clearly visible and dominates the internal space....its huge ! The pump is of a diaphragm type that employs two diaphragms working in 'flat twin' style opposition to each other. Each diaphragm has a 'cylinder head' that contains the inlet and outlet valves for the air. The air produced by the pump cylinders is fed to a air reservoir to reduce any pulsing in the output air stream. The air feed comes from the single outlet of the air reservoir.

The diaphragms in such a pump may be driven by a motor and associated cam shaft or the simpler, and often more reliable, vibrating armature that is often found in fish tank air pumps. The LF852D uses the latter design but it is far better engineered than a fish tank pump. The armature contains a large strong magnet that sits between two large field coils on laminated steel cores. The armature has quality diaphragm mounts at each end. The armature is attached to the diaphragms and these suspend the armature centrally between the field coils in all directions. The armature effectively 'floats' in the field created by the field coils. The intensity of the magnetic field is controlled by the 'Air Flow' control circuit on the front panel. The alternating field drives the magnetic armature laterally forcing the diaphragms in and out of the 'cylinder heads'. I have attached a Patent picture showing the basic idea.

This design of pump is very common and well understood. The armature does not suffer wear over time, unlike motor brushes and bushes. The diaphragms are a possible weak point if they perish but these are cheap and easy to replace. Another advantage of the design is that it can be very quiet in operation as there is little mechanical noise, just a thrumming sound from the air and diaphragm movement. Balanced double diaphragm pumps are preferable to the simple single diaphragm design that acts against a spring as the balanced design offers quieter operation.

My 'faulty' diaphragm pump hot air station had one of its diaphragms incorrectly seated causing poor operation of the armature. The original purchase had also neglected to remove the four transit screws so it would have been very noisy and rattled all over the place ! RTFM !

My PACE hot air station uses a high speed turbine air pump that produces a high quality continuous flow of air to the handpiece. The turbine pump is sold as a spare part by PACE as the motor can suffer brush or bearing failure with time. Like a lamp...it can 'burn' brightly but for a limited time ! As such it is considered a consumable.

The Pace desoldering stations use a pump that contains a diaphragm or piston that is driven by a connecting rod and crank shaft much like a conventional compressor design. The pump is often single cylinder and noisy when operating. The design also suffers the same operational limited life due to the use of a powerful brushed motor.

Some hot air stations use a brushless fan mounted in the handpiece. These appear to be an effective compromise in terms of design. A brushless 'snail shell' fan is capable of low noise air production and is only limited by the quality of the design and motor bearings. The motor is a weak point and bearinr failure is still a limitation, but the rliability is better than that of a brushed motor. The snail shell motor is not suited to applications that require any decent level of air pressure, as opposed to air flow (very different specifications). The size of hot air stations that use a small brushless blower may be significantly less than those that use a diaphragm unit and weight is also greatly reduced.

I trust that this information has been of interest.

Fraser

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