Air Compressors, Airless Paint Sprayers Buyer's Guide

A piston pump is a reciprocating pump in which two ball checks control the flow of fluid through the fluid section. A displacement pump rod connects the pump’s internal components to the drive system. The rod moves up and down-paint is loaded on the upstroke and displaced on both the upstroke and down stroke. “Piston pumps have improved over the past few years and the fluid pressure is now more constant,” says Peterson.

Piston pumps offer several advantages: they pump higher viscosity coatings with ease, and because they only run on demand, usually have lower maintenance costs. Piston pumps are generally more expensive to manufacture and have a higher initial price tag, but this pays for itself with the lower lifetime maintenance.

“A piston pump moves more material on each stroke and will be between 200 and 400 cycles per minute. This results in less wear and tear on the parts,” says Wood.

The traditional difference between a diaphragm pump and a piston pump was the loss of pressure or dead band-this is the pressure range between which a pump stops and starts supplying paint to the gun. It is a characteristic of all paint sprayers.

Take the example of a piston pump operating at 3,000 PSI designed to have a 500 PSI dead band. At one point, the pump will turn on and pump material into the line until line pressure reaches 3,000 PSI; then the pump will turn off. Even though the pump is now off, the line remains charged and ready to spray material. When the gun is triggered, the line pressure drops to the spraying pressure of, say, 2,000 PSI. As line pressure then drops further to 1,500 PSI, the pump will turn on again until line pressure once more reaches 2,000 PSI. The process will repeat itself, cycling between 1,500 PSI and 2,000 PSI while the gun is triggered. This 500 PSI is the dead band.

If the dead band is too large, it causes the phenomenon known as winking. The line pressure drops too far before the pump turns on to compensate, and the result is that the sprayed fan width fluctuates-or winks.

Sometimes dead band is just there and doesn’t really make a difference; at other times, especially when spraying thin coatings, dead band can be a real problem causing tails, winking, and poor atomization.

In the piston pump example above, a painter could spray standard latex paint without any problem. At full pressure (2,000 PSI), latex will atomize well; at 1,500 PSI, the other end of the dead band, some latex will still atomize satisfactorily. But if that painter switches to a thin coating such as a lacquer, it becomes a different situation. Lacquers typically atomize at around 800 PSI. Spraying it at 2,000 PSI uses too much force. In addition to the potential of over application, the lacquer will bounce back, contaminate the air, and waste a lot of material.

This is where electronic pressure controls come in. Recently introduced to better regulate spray pressure, electronic controls sense the drop in fluid pressure and send a signal to the circuit board. Recent advances in electronic controls have virtually eliminated dead band all together. The electronic controls can cycle the pump at exactly the speed required to support the spray tip at the pressure the painter selects. “Electronic controls regulate the pressure to deliver a consistent, fully atomized spray pattern at the tip at any pressure setting” says Dave Huml, worldwide product marketing manager contractor equipment division, of Minneapolis-based Graco Inc.

Sprayer motors can be operated from three different power sources: electric motor, gas engine or compressed air. In all electric and gas sprayers, the motor creates a rotating motion that is transferred to the drive source of the fluid pump.

In the case of motors powered by compressed air, the flow of the air through the motor causes the motor piston rod to move up and down: This reciprocating motion is then transferred from the connecting rod to the fluid displacement rod in the pump.

There are three different types of electric motors available on airless sprayers: AC, DC and Universal. The AC motor requires a continuous electrical power source and has the longest life of any motor currently used on airless sprayers up to 2,000 hours. AC works at 1,725 RPM and is found on most larger commercial machines.

The DC motor is more forgiving of power fluctuations because it can run at variable speeds, but the life of DC motors is rated at 1,750 hours compared to 2,000 hours of the AC motor.

The Universal motor has a big price advantage. These motors are similar to those found on drills and rotors, with life expectancies of only 300 resulting from running speeds of 10,000 to 20,000 RPM (three to five times faster than AC or DC motors).

Gas engines develop horsepower to drive the pump and have the advantage that they can be used on construction sites or areas where electricity is not available. Two types of drive systems are available on the gas airless sprayers. The gas engine develops horsepower to drive the pump and the electrical energy to power the clutch. This is known as a direct-drive system. It is 95 percent efficient-able to transfer up to 95 percent of the engine’s power to the fluid pump.

A hydraulic unit uses a gas engine to drive a hydraulic pump. The hydraulic pump draws hydraulic oil from a reservoir and pumps it to the hydraulic° motor. This causes the hydraulic motor to reciprocate and power the fluid pump. These steps cause hydraulic sprayers to be less efficient than a direct-drive system. “Accuspray’s design in hydraulics allow us to transfer 90 to 95% of the engines power to the pump and compensate for horsepower through our patented hydraulic pump. This allows us to achieve outputs of 3.5 GPM with a 188 lb., one man portable unit,” says Rodd Kaczmarek of Accuspray. Hydraulic units are 60 percent efficient, but because they typically use hydraulic pressure regulation the units have very good dead band.

Pressed to choose an ideal system on a limited budget, contractors can rely on the airless spray manufacturers for accurate up-to-date information on which to base a decision.