Many Donaldson off highway air cleaners contain two filters. The larger one is known as the outer or primary filter. The role of the primary filter is to remove contaminant from the incoming air as it passes through the air cleaner assembly.

There is often a smaller filter fitted either inside the larger pleated primary filter or after a PowerCore primary element. This smaller filter is often referred to as a secondary or inner filter. We prefer to call this smaller filter the safety filter as this better describes its function.

The safety filter has two jobs:

1. To protect the engine during routine changeouts of the primary filter. Every time the primary element is removed the safety filter protects the engine by preventing dislodged dust and contamination from entering the clean air side of the intake system.
2. To act as a backup in the rare case of transit or handling damage to the primary filter. This damage may not be apparent in a visual inspection. Additionally, it is possible that the primary filter could be installed incorrectly, preventing a good seal allowing dirty air bypass.

The safety filter usually uses a different type of media from the primary filter. The media is more open for lower restriction and is less efficient, meaning it won’t plug on exhaust soot and oil fumes but will still stop small, engine damaging, dirt and dust particles. Depending on which air cleaner the safety filter is designed for, they can have a pleated construction similar to the primary filter, or smaller footprint air cleaners may incorporate a ‘slim line’ safety filter design.

With less contaminant holding capacity, the safety filter will plug rapidly bringing on high restriction alarms when engine damaging particles that find their way past a compromised primary filter. If the safety filter is not changed at this time, the maximum restriction level will be reached again very quickly.

The overall efficiency of the air cleaner is not increased by the safety filter, and it is not a secondary filter—it is ‘insurance’ for your engine. Our experience has shown that safety filters are the most effective and efficient method of providing additional engine protection in high dust environments.

Many Donaldson air cleaners are designed to be fitted with a safety filter as an optional extra for that added piece of mind. Please contact Donaldson and we will advise you of the correct safety filter to use.

Did You Know?

It is recommended that the safety filter is replaced every second to fourth primary filter change-out. If contamination is obvious on a safety filter, investigate the cause of the dust ingress prior to changing the loaded safety element.

Never run an engine with only the safety filter installed.