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Understanding Laser Printer Mechanical Systems

Updated: Aug 21, 2019

Laser Printer Mechanical Systems


For definitions of terms in this article a glossary has been provide near the footer.

More is needed than just good electronics and a fancy case to make a first-class laser printer. A series of tightly integrated mechanical systems are needed to perform the verity of physical tasks that every laser printer must do. The most obvious mechanism is the paper transport system responsible for moving paper through the printer, but the image formation system also needs mechanical force to function.

Laser printers need a well-regulated scanner motor assembly that can direct a laser beam across a light-sensitive drum surface. In previous articles you we discuss the selection of important mechanical parts in but this article shows you how to deal with mechanical problems.


Troubleshooting Laser Printer Paper Problems

Before you jump right into paper detection and handling symptoms, you should have a solid understanding of how paper is handled in a typical laser printer. Paper in a paper Tray is loaded into the printer. A paper-detect sensor makes sure that paper is available in the tray, and the tray itself actuates a series of tray size micro switches.


Each tray size actuates a different sequence of switches that tell the logic assembly just which tray size is now installed, which allows the printer to automatically size the image according to the paper tray being used.

When a printing cycle begins, the main motor turns, which causes mechanical linkages to turn the toner drum, fusing rollers, and any feed rollers that carry the paper along. The only two mechanisms that do not turn are the paper pickup roller and the registration rollers. Their actions are regulated by solenoid-driven clutches that remain open.

HP Laser Printer Technician Basics


When the printing cycle is ready to receive a page, the pickup roller clutch engages. The notched pickup roller grabs the top page and draws it into the printer about 7.5 centimeters (about 3 inches), which is about the circumference of the pickup roller. After one turn, the pickup-roller clutch disengages, and the page rests against the registration rollers. A rubber separation pad just below the pickup roller prevents more than one page at a time from entering the printer.

When the developed drum image is properly positioned in relation to the page, the registration roller clutch engages and starts the page into the image formation system. Once the registration rollers start, they will remain engaged until the paper has exited the printer.


Feed rollers guide the page while the latent image is transferred to it, then gently transfer the page to the fusing assembly where the toner image is fixed. As the paper emerges from the fusing rollers, a paper exit sensor is actuated. When the page has left the printer, the sensor assumes its original condition, and the printer logic assembly knows that the page has left the printer. The main motor can now stop (or a new page can enter the printer for another cycle).


Introduction To HP Laser Printer Repair and Troubleshooting


Symptom 1

You find a PAPER OUT message. When the printer generates a PAPER OUT message, either the paper tray is empty or the paper tray has been removed. When a paper tray is inserted, a series of metal or plastic tabs contact a set of microswitches. The presence or absence of tabs will form a Paper tray code that is unique to that particular paper size.


Microswitches are activated by the presence or tabs. Main logic interprets this paper type code, and knows automatically what kind of media (paper, envelopes, etc.) that it is working with.


Table 8-1 shows a typical laser printer paper tray code. Tray type SW1 SW2 SW3 Executive 1 1 1

A4 1 1 0

Legal 0 0 1

Envelope 0 1 1

Letter 1 0 0 No tray 0 0 0 1 = on (engaged).

0 = off (disengaged).

The presence of paper is detected by a mechanical sensing level. When paper is available, a lever rests on the paper. A metal or plastic shaft links this lever to a thin plastic flag. While paper is available, this flag is clear of the paper out sensor.


If the tray becomes empty, this lever falls through a slot in the tray, which rotates its flag into the paper-out sensor, which shows that paper is exhausted, The paper-out sensor is usually mounted on an auxiliary PC board (known as the paper control board).


Begin your check by removing the paper tray. Be sure that there is paper in the tray, and that any ID (identification) tabs on the tray are intact especially if you have just recently dropped the tray. Re-insert the filled paper tray carefully and completely. If the PAPER OUT message continues, then there is either a problem with your paper tray ID microswitches, paper sensing lever, or the paper out optoisolator.


Troubleshooting HP Laser Printer Power Problems

You can check the paper ID microswitches by removing the paper tray and actuating the paper sensing lever by hand (so the printer thinks that paper is available), Refer to Table 8-1 and actuate each switch in turn ate one switch at a time and observe the printer display.


The PAPER OUT error should go away whenever at least one microswitch is pressed. If the error remains when a switch is pressed, that switch is probably defective. Unplug the printer and use your multi meter to check continuity across the suspect switch as you actuate it. Replace any defective switch or switch assembly. Inspect the paper-out lever and optoisolator next