Dispensing systems for optimal lubricant metering
The four key factors for successful system design
Around 1,000 different components are installed in a vehicle during the production process. Then there are more than 150 common greasing and oiling applications, although this can be a lot more depending on the vehicle type. They enable window regulators and sliding roofs to move, make knobs easy to turn, and ensure that the engine and transmission work reliably. The application for lubricant dispensing is therefore always directly connected to the end product and its intended use. This defines the functional requirements for the lubricants used, such as greases and oils:
- Ease of motion, reduced friction
- Error prevention
- Continuous operation and durability
- Noise reduction and prevention (anti-squeak)
- Tolerance compensation (anti-rattle)
The requirements for the type of lubricant dispensing vary and always depend on the application being carried out. There are many factors involved in this: Should the application be carried out in dot, bead or spray form? In what timeframe should the application be carried out? What is the possible deviation in the applied amount? How is the metering system integrated into the production line? The conditions for these requirements generally specify the manufacturing process in question in the automotive industry. For these, application cases for metering technology involve:
- Short cycles, high repetition rates
- Defined tolerance ranges
- Documentation of process capability
- Confirmation of repeatability
- Use of complex media (separation: explanation below)
- From 0.003 ml to time-controlled filling (point to volume filling)
- Direct or indirect control (cylinder movement, flow measurement, light barrier) depending on the product selected
Clarifying the metering requirements
These characteristics form the basis for configuring a appropriate metering system and establishing the metering requirements. Four key factors can be identified to ensure optimal results with maximum precision and repeatability:
1. What is the medium in question?
2. How can the metering process be described?
3. What room and ambient temperatures should be considered?
4. What are the requirements for control and documentation?
These key factors are not always clear initially, which is why a detailed examination or investigation is a good idea to ensure successful project execution.
The material: defining the properties
First, the detailed material data of the lubricant should be determined, as knowledge of its specific physical and chemical properties is vital to arrive at a correct assessment of the application and the resulting system design. This includes information about viscosity, the specific behaviour – whether the lubricant is abrasive or contains fillers – and its separation.
In general, most lubricants can be processed without any metering problems. It is particularly important to consider the chemical compatibility with sealing materials. Depending on the base material and the addition of additives, the resistance of the seals for pumps and metering agents must be tested when conveying and metering greases and oils. Mineral-based oils with high aroma content in particular often show increased seal incompatibility. For example, some types of grease tend to bleed easily under high pressure. This type of separation can occur when the soapy, liquid components and the solid components of the medium come apart, for example due to high pressure in the container or in the pump. This can result in functional failure, as the oil escapes from leaking system components and the thickening agent remains inside in compressed form. In this case, the effect for greasing would no longer be optimal and pressure regulation should be carried out.
Viscosity and consistency classes
Viscosity, or the flow behaviour of the material, also plays a role. It has a huge effect in relation to oils. The viscosity ranges from water thin to ‘just flowable’. For example, silicone oils used in vibration dampers are so viscous that they will hardly flow out of an upside-down cup. Compared to oils, greases have a significant advantage due to their consistency: they can remain at the friction point. Greases generally consist of two basic materials: oils and thickening agents. There are essentially three properties involved in greases: the base oil type, the thickening agent type and the consistency.
The consistency of a grease is essential for the application and should vary as little as possible at different temperatures. The overall consistency of a grease depends on the viscosity of the base oil and the use of the respective thickener type and its physical properties, which in turn depend on the requirements of the application.
Only a precise determination of the flow behaviour or viscosity can establish whether a lubricant is suitable for the specific application, the extent to which it can be pumped without difficult and what metering technology can reliably implement the desired grease or oil application.
Outlining the process description
In the simplest situation, an application involves a pump, a valve and a pressure control. In reality, however, the lubrication mechanisms are integrated into complex, fully automated production lines. A detailed description of the process and production layout is therefore vital in the design of a metering system. A specific description of the application involves the following questions:
What is the application in question?
- How is the application carried out?
- What is the volume being produced?
- What is the cycle time, i.e. the time within which the lubricant needs to be metered and applied?
- How are the materials supplied?
- What is the distance between supply and application of the grease?
- What pressure is required for processing?
There are usually only two options for positioning a metering system. Either the part to be greased is positioned or the metering valve has to move towards the part. The specified cycle time also plays an important role in the design of the system, in other words the time that is specified for the metering process. The duration of the grease application must always be considered in relation to the cycle of the system. With this in mind, calculations should be made in advance and the cycle times should be specified precisely in order to configure the system efficiently based on these specifications.
As shown in the observation on the consistency classes of greases, this should remain as constant as possible at different temperatures in order to obtain optimum metering results. The processing temperature is always compared to the ambient temperature. Example: if the ambient temperature is approximately constant, for example at an assumed 40°C, the material should be heated steadily to a higher temperature, here to 42°C. In this case, however, it should also be ensured that the material does not change with the increase in temperature, and if it does, that its quality does not deteriorate. Reproducibility can only be ensured by observing the ambient and processing temperature as well as possible temperature compensation and by checking the material quality.
Control and documentation
Short cycles and high repetition rates in the production process also place high demands on process and repeatability control. There are different ways of providing evidence of successful metering. One way is for it to be clearly traceable that the grease or oil has been metered. This indirect control is usually carried out via Yes/No or 1/0 documentation. A sensor system and metering unit control can therefore be used to document whether metering has taken place. This enables the smallest quantities to be presented consistently. Indirect control can be carried out using a camera or a light barrier, for example. The requirements for control and documentation are usually defined and should be communicated in the planning process for the metering system. Depending on the type of evidence that is required, this should be incorporated into the planning.
Material properties, process description, temperature, and control and documentation requirements – by analysing these four key factors and incorporating them into the system design, the main requirements for the metering system can be met: precise, reproducible metering of grease and oil. This is the only way that auxiliary and wear lubrication in vehicles fulfil their main functions: smooth running and friction reduction, error prevention, continuous operation and durability, noise reduction and avoidance (anti-squeak), and tolerance compensation (anti-rattle).