HOW DO THE MARINE ENGINE MOUNTS LOSE THEIR ELASTIC PROPERTIES

This article is associated to a precedent article that deals about the symptoms of how to detect when an flexible engine mount of a marine engine has to be replaced.

On this article we are putting more weight on the reasons and process of degradation of the elastomeric compound in marine engine mounts. Knowing how and why the elastic properties of the engine mounts degrade, we believe that this article can help to the boat owners which practices are good to prolong the life of the marine engine mounts.

This article is applicable for all kind of marine engines, no matter if the brand is Yanmar, Volvo, Mercury, Beta marine or other. The degradation process on the elastomer happens on all type of marine engine mounts on similar manner no matter the engine brand or model.

We will start first by the mechanics of the elastomers. No matter the rubber type, hardness, colour, all elastomers are composed of polymeric chains. The term polymer comes from the Greek “poly” meaning many and “mer” meaning parts. Natural rubber as well as other rubber compounds is a polymer, a long, chain like molecule that contains repeating subunits.

Representation of a subunit of NR

Representation of a subunit of NR

One can make use of the analogy of a plate of tangled spaghetti representing a polymer mass. The individual spaghetti represents a single polymer chain. The long chain length allows for entanglement. With the discovery of vulcanization, a structure could be formed with sulfur bonds linking individual polymer chains into a 3-dimensional network. Chains now have extensibility which allow for the support of stress and a retraction upon the release of the stress. Our spaghetti analogy has just turned from unconnected spaghetti stands to a fishing net structure as in vulcanized rubber.

Representation of a sulfur cross links between NR strands blue and green.

Representation of a sulfur cross links between NR strands blue and green.

So, in a simplistic way we can define that the Elastomers are composed of an immense set of chains, like shown on the below picture.

Vibrations on a marine engine can occur for several reasons or origins, engine unbalance, sea conditions, amplification due to resonance with rotating frequency of the engine, misalignment of the propeller shaft or a combination of them. Vibrations create stress and strain on the rubber like it can be seen on the below fatigue test video.

This stress and strain creates a tension on the polymeric chains. The tension on the system is shown on the below FEM image.

During the years the polymeric chains are subjected to many strain stress cycles. The polymeric chains will break proportionally to the number of cycles.

On the Fig 3 it is represented a load vs deflection chart of two marine engine mounts, one being brand new and the other one being used. As indicated above, the strain and stress caused by the dynamic loads and vibrations on the elastomer, polymeric chains break. Therefore, the marine engine mount with the age, shows a lower amount of polymeric chains to withstand the same load. This affects on the deflection of the mount. As it can be seen, with the time, the marine engine mount deflection goes from S1 to S2. This is because the remaining polymeric chains have been resisting as much as they could but obviously deforming more.

Load vs Deflection curve on a new mount and a used mount.


Load vs Deflection curve on a new mount and a used mount.

From an isolation point of view we need to understand that the stiffness of the flexible engine mounts plays a key role on the isolation. But what is the stiffness? The stiffness is the proportion between force and displacement. This is to say, the amount of force that is needed to provide a given displacement or deflection. The stiffness is represented with a dotted brown line, showing the proportion or slope of the curve at a give force (F1). The stiffness 0 is the stiffness of the new mount and the stiffness 1 is the stiffness of the used mount. Stiffness of the used mount is higher as the new one.

The stiffness plays a major role on the isolation of the engine. It determines the resonant frequency of the system. The higher is the stiffness of the suspension the higher the natural frequency, so lower will the isolation be.

Natural frequency formula where K is stiffness of the mounts and M is the mass of the engine

Natural frequency formula where K is stiffness of the mounts and M is the mass of the engine

So, in other words even if the engine moves more and the system is showing more elasticity, one could think that the engine is better isolated against vibrations, but the case is just the opposite. The vibrations are felt higher than ever.

Since the system is more elastic, the misalignments of the shaft are more pronounced. Below video shows a comparison between used and new mounts.

 

In the video below you can see a part with elastic properties degraded by time.

 

Summary:

The degradation of the elastomers happens in all marine engine mounts, their degradation is directly dependent from the load cycles and their magnitude. The degradation comes from the loss of polymeric chains. The lower the polymer chains we have on the mount, the more elastic the system will be. The more elastic the marine engine is, the more movement will show the engine, apart from bending shafts and unwanted noise, this will cause higher stress on the mounts which will lead to break the remaining polymeric chains. Creating a self-feeding degradation.