# Hammer mill glossary URL: https://moinhosvieira.com.br/en/hammer-mill-glossary/ Updated on: 2026-11-03 Other languages: Español: https://moinhosvieira.com.br/es/glosario-molino-de-martillos/ · Português: https://moinhosvieira.com.br/glossario-moinho-de-martelo/ > Short definitions of the terms used on Vieira hammer mills, from the grinding chamber to the starter, with links to the page on each subject. ## Grinding - **Hammer mill**: A machine that reduces a product to smaller particles by the impact of hammers mounted on a rotor that spins inside the grinding chamber. On Vieira mills the system is centrifugal and pneumatic: the product is drawn into the chamber by suction and carried by air to the cyclone. See [our hammer mills](https://moinhosvieira.com.br/en/our-mills/). - **Grinding chamber**: The chamber that houses the rotor, the hammers and the screen, where the product is ground through 360°. One grinding chamber: MCS series and MCO series. Two: MCD series. - **Pre-grinding**: The first grinding stage on double grinding mills (MCD series): the product is ground in the 1st chamber, the smaller one, and moves through the connecting pipe to the 2nd chamber. - **Final grinding**: The second stage on double grinding mills (MCD series), in the 2nd chamber, the larger one. The screen in that chamber sets the final particle size; the product then goes to the cyclone and to bagging. - **Particle size**: The size of the ground particles. It is set mainly by the screen hole: a smaller hole gives a finer product. If the particle size changes without a screen change, check the [screen guide sealing rubber](https://moinhosvieira.com.br/en/replacement-of-screen-guide-sealing-rubbers/). - **Mesh**: A measure of how fine a screen is, by the number of openings per inch: the higher the mesh, the finer the product. Grinding reaches up to 120 mesh on the MCS 280 and MCS 350 models and up to 100 mesh on the MCS 190 and MCD series models, depending on the product. See the conversion table on [screens for mills](https://moinhosvieira.com.br/en/screens-for-mills/). - **Rotational speed**: The speed of the machine shaft, in revolutions per minute (rpm). In the model specifications it ranges from 2,500 rpm (MCD 950) to 6,000 rpm (MCS 280 and MCS 350). Compare on [all mills](https://moinhosvieira.com.br/en/our-mills/). - **Working area**: The floor space the mill needs to operate; on models with a cyclone, not counting the cyclone. The full installation, including the cyclone and the bag filter, is longer: see each model's area and length in [how to choose a hammer mill](https://moinhosvieira.com.br/en/how-to-choose-a-hammer-mill/). ## Rotor and hammers - **Rotor**: The part fixed to the shaft that spins inside the grinding chamber and carries the hammers, mounted on pins with spacers. MCD series mills have two rotors: the smaller one for pre-grinding and the larger one for final grinding. - **Hammer**: The part that strikes the product. Replacement hammers are supplied weighed in pairs to keep the rotor balanced. See [hammers for mills](https://moinhosvieira.com.br/en/hammers-for-mills/). - **Working edge**: The edge of the hammer that strikes the product. When the mill's output drops, the hammers are turned to work with a new edge, without replacing pins or spacers. See turning the hammers. - **Pins and spacers**: The pins hold the hammers on the rotor and the spacers keep the distance between them. They are replaced every 5 hammer replacements. See [rotor pins](https://moinhosvieira.com.br/en/video-about-rotor-pins/). ## Screen - **Screen**: A perforated sheet that surrounds the rotor through 360° and sets the particle size. It fits into the guide at the bottom of the grinding chamber. Holes from 0.3 to 6 mm in the capacity table. See [screens for mills](https://moinhosvieira.com.br/en/screens-for-mills/). - **Screen frame**: The structure that holds the screen. It is part of the screen replacement set, together with the screen itself. See [hammer mill parts](https://moinhosvieira.com.br/en/parts/). ## Feeding, cyclone and bagging - **Suction feeding**: The inlet of the grinding chamber works under negative pressure: the product enters together with part of the air the mill draws in. That is why feeding must be gradual. Too much product chokes the mill and makes it push product back; see the [troubleshooting guide](https://moinhosvieira.com.br/en/hammer-mill-troubleshooting/). - **Feed hopper**: The funnel where the product is loaded to enter the mill. The feed hopper comes with the MCS series, MCD 530, MCD 680B and MCD 680A models. - **Outlet chute**: The part at the outlet of the grinding chamber that leads the ground product into the pipe. - **Bagging cyclone**: Receives the ground product through the mill–cyclone pipe and discharges it into the bag through the bagging valve. The cyclone comes with the MCS series and MCD series models; the MCO series has no cyclone. It is made of polyethylene with a tripod on the MCS 280, MCS 350, MCD 530 and MCD 680B models. See the [mill–cyclone pipe](https://moinhosvieira.com.br/en/cilclone-mill-connection-pipe-installation/). - **Bag filter**: A fabric filter fitted to the bagging cyclone, through which the air that carried the product leaves. The bag filter housing with its filter bags comes with the MCS series and MCD series models; the MCO series does not have one. See [bag filter installation](https://moinhosvieira.com.br/en/installation-of-the-bag-filter/). ## Electrical and maintenance - **Star-delta**: A type of starter that connects the three-phase motor in two stages, first in star and then in delta, for a smoother start. It is one of the suggestions for the MCS 280, MCS 350 and MCO series models. - **Soft starter**: An electronic starter that accelerates the motor gradually. It is the recommended starter for the MCD series, and also one of the suggestions for the MCS 280, MCS 350 and MCO series models. The electrician defines the right model. - **NLGI 2**: A grease consistency grade on the NLGI scale. The recommended bearing grease is lithium soap NLGI 2, applied every 30 days or 200 working hours. See [bearing lubrication](https://moinhosvieira.com.br/en/bearing-lubrication/).