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HP Laser Printer Mechanical Assemblies - Exploring And Troubleshooting The Printer Path Path.

Updated: Jun 10, 2019

Understanding the basic mechanics of a HP laser printers is the very first step towards successful troubleshooting. The following is designed to give a basic understanding of each printer assembly and its primary functions – to save you a lot of time and money troubleshooting and ordering HP printer parts.

HP Laser Printer Mechanical Assemblies - Exploring And Troubleshooting The Printer Path Path.
HP Laser Printer Mechanical Assemblies - Exploring And Troubleshooting The Printer Path Path.

Remember: while not every printer is the same, each model has the same basic assemblies in some way, shape, or form. The Cassette Pickup Assembly Depending on the model of the machine, “the pickup assembly” could just be a pickup roller with a metal shaft and clutch, or a series of clutches, gears, and rollers. Regardless of how different they look, they all perform the same basic function.


Typical HP printer picks paper up from the paper tray and transports it to the registration assembly through a series of clutches, gears, and rollers. Printers have been getting much faster and more complex throughout the years.


The HPLJ II/III series pickup assembly consisted of a clutch, shaft and gear. The HPLJ 4000 series has a pickup assembly and a feeder assembly, and the HP LJ 5si and HPLJ 8000 models have the pickup and feeder assemblies integrated into the Paper Input Unit, or PIU.


HP Laser Paper Input Units PIU

Some paper input units have their own motor within the unit to drive the assembly. The rest are driven off the printer’s main drive assembly. The “pickup assembly” is responsible for two things: its first function is to pick up paper. Its second function is to prevent multi-feeding, through the use of a separation pad, separation roller or wear strips.


The goal of this assembly is to get one sheet of paper to the feeder assembly. In most printers, this assembly has a sensor to let the machine know that the tray is out of paper. Some models do not have this sensor, and the machine will give a paper jam error when the tray is empty.


This is because it assumes there is always paper in the tray, and the first sensor does not get tripped. The feeder assembly has one function. It assists in getting the paper to the registration assembly from the cassettes. On some of the smaller machines, the pickup roller also acts as the feed assembly. This is due to the more compact design of the machine.


This assembly is also used in machines that duplex. The duplexer feeds the paper from its output to the input of the feeder assembly to print the second side of the paper. Many machines with paper feed assemblies have a jam sensor in the unit.



HP Laser Printer Manual Pickup Tray / Multi Purpose Tray Pickup Assembly

The Manual Pickup Tray The multi-purpose tray pickup assembly uses a slightly different paper path, but its function is simple: get the paper from the tray to the registration assembly. This tray works very well for labels or media that need a more direct paper path (such as thicker media), along with odd-sized paper. Most of the time, media in the tray is recommended to exit out the face-up bin on one side of the machine, depending on the model.


This assembly has a sensor to inform the DC Controller that there is paper in the tray. If you forget to plug in this sensor, the machine will do one of 2 things. Either it will always think there is paper in the tray, or it will never think there is paper in the tray.


If your machine has a control panel and keeps prompting for paper in the MP tray, check what paper size and types are in the cassettes. This can be done by printing a configuration page. If none of the cassettes match what media the print job is asking for, or the print job is requesting to be printed from the manual feed tray, the unit will prompt the user to put the selected media in the MP tray.


This assembly also has either a clutch or solenoid to engage the assembly. When the controller sends a signal to pick paper from the MP tray, it engages the solenoid or clutch. This causes the pickup roller to turn the separation pad to lift and paper to be picked up from the MP tray. From there, the paper is fed into the registration assembly.

The Registration Assembly The registration assembly has one function.


This is to ensure that the paper is going through the image formation process straight so the image is not skewed. This assembly can be as simple as a few offset rollers guiding the paper to a straight-edge on the side of the paper path, to a set of rollers with a metal guard operated by a solenoid or clutch. Once the paper passes this assembly, the top of page sensor is tripped.


This sensor is located right after the registration assembly. When this sensor is tripped the image formation process begins. If the registration is not functioning properly, paper will either jam in front of it or the image on the page will not be centered properly. If the top of page sensor is missing or not tripped, you will get a blank page, followed by a paper jam in the fuser.


This is because the fuser jam sensor tripped, yet the controller is still waiting for the top of page sensor to be tripped. Image Formation After going through the registration assembly, the paper is sent to the imaging section of the printer.


This consists of the toner cartridge/ transfer roller sections, the feed belts (if applicable), and the fuser. When paper passes between the toner cartridge and the transfer roller, the toner is transferred from the cartridge to the paper, and pushed along.


If the transfer roller is not installed properly or the springs are missing, you will get an uneven print density on one side of the page. But we can explain this in more detail later. Depending on the machine, the registration assembly is doing the advancement of the paper during this process until it reaches the fuser. Smaller media uses feed belts to transport paper between the image transfer and fusing section of the printer in some machines.


When the paper enters the fuser, it is squished between two rollers to fuse the toner to the paper. The paper then exits the fuser rollers, where it goes out the delivery rollers. Between the fuser rollers and the delivery rollers is a jam sensor. This sensor is there to inform the controller that paper is in the fuser. If this flag is tripped when it shouldn’t be, it tells the printer's DC Controller to not turn on the fuser.