Haris: Hello Martin, how are you?

Martin: Good afternoon Haris, how are you doing?

Haris: Excellent, new Monday, new ice cream, new coffee, and new topic obviously!

Martin:Funnel-y” enough, yes… a new topic.

Haris: I was thinking about discussing viscosity today, what do you think about it?

Martin: Well, again…”funnel-y” enough, I thought of that and, ice cream is something you can pull through a funnel unless of course it’s melted and it’s onto your shirt.

Haris: That is not ice cream anymore.

Martin: Yes, that’s true and the local neighbour dog is licking your shirt. Anyway, so, talking about funnels got me thinking about viscosity again and, you know, a lot of us may be using a funnel something like this out on site. This is obviously quite a big Funnel designed for oil to go through but, you know, as you pull that oil in there, we find that the oil level rises up and then we have to stop, let the oil go down again. But what we are physically watching at that point is the essence of kinematic viscosity because, by definition, kinematic viscosity is a measure of the oil resistance to flow and shear with respect to the forces of gravity. As engineers, we often talk about centistokes as the unit of measure but, in actual fact, the correct unit of measure would be millimetres squared per second which is what you find when you look at the data sheets for your oils and greases, they quote the viscosity in millimetres squared per second. However, viscosity it’s quite a key factor because it’s probably the most crucial property of any lubricant. Because it affects not just the wear rate, but it affects the power consumption as well and if we have the wrong grade or the wrong viscosity of oil, if that viscosity is too low, it’s unable to provide the full strength to support the load and we’re going to end up with metal-to-metal contact which, of course, means then increased wear rate. On the other hand, we come back to that issue of drag, when the viscosity is too high, then, of course, we have to overcome that drag, that thicker oil, and we have to put more power into the system to operate it correctly. So, it’s kind of a balancing act and adding the fact that we also have to take a number of other factors into account, you know, a machine designed for operation in a cold country may not use the same viscosity grade of oil in a hot country, simply because the ambient temperatures is much higher, so we need a viscosity grade of oil that will have us positioned at the right level for the ambient temperature.

So, actually very very difficult, but you know I’m sure you’ve seen this on site as well. There’s an awful lot of consolidation and rationalizing of lubricants and so you get these decisions to perhaps… put all the gearboxes on a 220 grade oil rather than having them on a 150, 220, 320 etc. etc. as per the OEM requirements or we find that there’s been drift away from the original viscosity because somebody decided to try something a bit thicker because it was running a bit hot and forgot to noted down in the management change process and so, you know, and then you’ve got the guesswork onside, people going: ‘oh, I’m going to change the oil in this area today, I’ve got to change oil on those gearboxes. What are we using those gearboxes? Oh, it’s probably the 460 because that’s what I use in my gearboxes, in my area.’

Haris: Yes, this is something that we mentioned very often, it’s ‘a friend told me’, a logic in lubrication, because there is a guy doing it quite well so, he could have just given me an advice. And, again, it doesn’t really work, because it’s such a fine balance job, that if you miss it, by a little, the consequences will be huge as usual.

So, what else is very interesting and specific about viscosity? what else we should tell our viewers and friends to take care about.

Martin: Well, the other thing is to look at the viscosity index because the viscosity index gives us a feel for how much the viscosity changes as the temperature rises and, again, going back to living in cold countries, when you’ve got these cold start issues, you want an oil that has a viscosity grade that’s capable of still lubricating, that’s not going to prevent the machine starting when it’s extremely cold. But, at the same time, as the machine warms up, you don’t want that oil filling out too heavily. So, viscosity index is also a factor to consider and I often see oils that have the letters HVI as part of the product name, often with hydraulic oils, for example, and what that will typically mean is high viscosity index. So, again, the clues often are in the name of the product, the HVI alluding to the high viscosity index which will be higher than your typical mineral oils and remember as well with synthetic base oils they do naturally have a higher viscosity index, we can, of course, improve the viscosity index on mineral oils through the use of viscosity index improver additive and that was very common in automotive oils with mineral base oils some time ago, but, of course, nowadays, most of manufacturers are calling for fully synthetic base oils and they naturally have that higher viscosity index. But, you know, your passenger car is a classic example of starting up on a cold morning and then the engine getting up to temperature and having to have the right viscosity index as well as the right viscosity grade.

Haris: Yes, I’ve seen such logic cases and it was really marvellous how easy is make mistakes. When a machine is stopping at -37 and then it takes 45 minutes to get on the working temperature and it started to stop several times a day. But I had the pleasure to see also the completely other way around, in a very hot area, especially in the cooling towers, when a gearbox is not operating the ambient temperature is extremely high, so it’s basically colder when it’s working, so it’s the other way around. We definitely need to pay a very very big attention to that.

Martin: Yeah, sure.

Haris: Thank you for this discussion, it was very very insightful and I’m very happy we discussed this. Should we meet next Monday?

Martin: It sounds like a plan again Stan!

Haris: You got yourself a date! See you on Monday.

Bearing Lubrication Monitoring
Lubexpert