Open gear lubricant film thickness analysis is a technical method for determining whether a lubricant can develop an adequate protective film between the teeth of heavily loaded open gears. For large gear sets used in mining, cement, power generation and other industries, understanding film thickness can help maintenance and reliability teams evaluate lubricant selection beyond basic properties such as viscosity or adhesion.
The analysis is based on elastohydrodynamic lubrication (EHD) principles. It considers the characteristics of the gear set, operating conditions and lubricant to calculate the expected lubricant film and Lambda value.
This matters because insufficient film thickness can increase the likelihood of metal-to-metal contact and wear. Petron Corporation notes that AGMA and ISO recognise the importance of selecting the correct open gear lubricant and provides EHD Film Thickness Analysis for specific gear sets.
What does lubricant film thickness mean?
Lubricant film thickness is the thickness of the lubricant layer separating two surfaces in relative motion.
In an open gear, the lubricant is applied to the gear teeth before they enter the contact zone. As the teeth mesh under load, pressure increases dramatically within the contact area. The lubricant film must be capable of separating the opposing surfaces sufficiently to reduce direct metal-to-metal contact.
The objective is not simply to have a visible layer of lubricant on the gear. What matters is whether the lubricant can generate an adequate film under the actual pressure, speed, temperature, geometry and load conditions of the gear set.
This is why a lubricant that performs well on one open gear cannot automatically be assumed to provide the same film performance on another.
An open gear lubricant film thickness calculation provides a way to evaluate this relationship quantitatively.
What is EHD film thickness analysis?
EHD film thickness analysis is a calculation used to estimate the lubricant film generated between contacting gear surfaces under elastohydrodynamic lubrication conditions.
Elastohydrodynamic lubrication occurs when the pressure generated between contacting surfaces is high enough to elastically deform those surfaces while also increasing the lubricant’s resistance to flow. These effects allow a lubricant film to form between the surfaces even under substantial loads.
For open gears, EHD analysis is useful because it moves lubricant evaluation beyond a product specification sheet. Instead of asking only:
“What is the lubricant’s viscosity?”
the analysis asks:
“Given this lubricant, this gear geometry and these operating conditions, what film can be expected at the gear contact?”
Petron’s EHD Film Thickness Analysis uses a proprietary computer program to calculate film thickness and Lambda for a specific gear set. Petron states that the program uses the same complex calculation approach described in AGMA and ISO standards while simplifying the information required from the user.
Why does lubricant film thickness matter?
Lubricant film thickness matters because an adequate film can help separate gear tooth surfaces and reduce metal-to-metal contact and wear.
Petron’s EHD resource points out that ring or girth gears are typically designed for at least a 25-year service life, yet some gears and particularly pinions can fail much earlier. Petron identifies wear as the most common cause of ring-girth gearing failures discussed in its resource and explains that increasing service life requires reducing or eliminating wear.
A lubricant can contain friction modifiers and other additives that provide boundary lubrication during conditions such as startup or very low Lambda values. But when operating conditions allow sufficient film formation, full EHD lubrication can provide a greater degree of surface separation.
This distinction is important.
A lubricant can appear to cover a gear tooth adequately while still producing an insufficient film at the actual contact point. Conversely, a lubricant selected using an EHD analysis can be evaluated against the specific operating conditions of the gear.
What is Lambda in EHD analysis?
Lambda, commonly represented by the Greek letter λ, is a dimensionless parameter used to compare lubricant film thickness with the combined surface roughness of the contacting surfaces.
Conceptually:
Lambda = lubricant film thickness ÷ composite surface roughness
A higher Lambda value generally indicates greater separation between the opposing surfaces, while a low Lambda value indicates that surface asperities are more likely to interact.
The exact interpretation should be made in the context of the calculation method, gear condition and operating regime. Surface finish, tooth condition and actual operating conditions all influence the result.
For this reason, EHD analysis should not be reduced to a single “good” or “bad” number without considering the wider engineering context.
What variables affect open gear lubricant film thickness?
An open gear lubricant film thickness calculation depends on several variables associated with both the lubricant and the gear set.
Petron’s EHD analysis request identifies a number of the inputs required to perform its calculation, including gear and pinion tooth counts, pinion speed, gear face width, power, tooth pitch, pinion tooth temperature, gear type, pressure angle, tooth condition and the current lubricant being used.
The key factors include:
1. Gear speed
Speed affects how lubricant is drawn into the contact zone. Changes in rotational speed can therefore influence the film that develops between the gear teeth.
2. Load and transmitted power
Higher transmitted power generally means greater forces acting through the gear mesh. The lubricant must maintain an appropriate protective film under those operating conditions.
3. Lubricant viscosity
Viscosity is an important contributor to film formation. However, selecting a lubricant solely because it has a high viscosity does not guarantee the required EHD performance.
4. Tooth temperature
Temperature affects lubricant viscosity and therefore influences film formation. This is particularly important on large industrial gears where operating temperature can differ substantially from ambient temperature.
5. Gear geometry
Tooth pitch, pressure angle, number of teeth, face width and other gear characteristics influence the contact conditions and therefore the EHD calculation.
6. Surface condition
The condition of the gear and pinion teeth affects the relationship between film thickness and surface roughness. Worn or damaged surfaces can change the lubrication regime even if the lubricant itself has not changed.
7. Lubricant characteristics
The properties of the selected open gear lubricant influence the calculated film thickness. This is why comparing products using the actual gear-set parameters can provide more useful information than comparing generic viscosity values.
How is open gear lubricant film thickness calculated?
Open gear lubricant film thickness is calculated by combining lubricant properties with the geometry, speed, load, temperature and surface characteristics of the gear set using an EHD calculation methodology.
A simplified process looks like this:
Step 1: Collect gear-set information
The analysis begins with information about the pinion and gear, including tooth counts, pitch, face width, pressure angle and gear type.
Step 2: Establish operating conditions
The calculation requires information such as pinion speed, transmitted power and tooth temperature.
Step 3: Identify lubricant characteristics
The lubricant being evaluated is entered into the analysis so that its relevant properties can be incorporated into the calculation.
Step 4: Evaluate contact conditions
The calculation considers how the gear surfaces interact under the specified operating conditions and determines the expected EHD film thickness.
Step 5: Calculate Lambda
The resulting film thickness can then be compared with the surface characteristics of the gear teeth to determine the Lambda value.
Step 6: Compare the result with operating requirements
The calculated result can be used alongside equipment condition, OEM requirements and maintenance history to determine whether the existing lubricant is appropriate or whether another product should be evaluated.
This is fundamentally different from using a generic open gear lubricant film thickness calculator that considers only a few lubricant properties. A useful engineering analysis needs to account for the actual gear set and its operating conditions.
EHD film thickness calculator vs. application-specific analysis
A search for an EHD film thickness calculator or open gear lubricant film thickness calculator often reflects a desire to compare lubricants quantitatively.
However, a calculator is only as useful as the variables it considers.
For example, simply entering lubricant viscosity and temperature cannot adequately describe a complete gear contact. The result can change when gear speed, load, tooth geometry, tooth temperature or surface condition changes.
An application-specific EHD analysis is therefore more useful when the objective is to make a lubricant-selection decision for a critical open gear.
Petron’s approach is designed around this principle. Its EHD program requires specific gear-set and operating information and calculates Lambda for the application. Petron states that its program was developed to simplify the required inputs while retaining the complex calculation methodology described in AGMA and ISO standards.
What role do AGMA and ISO standards play?
AGMA and ISO standards provide recognised methodologies relevant to gear design and lubrication analysis.
Petron states that both the American Gear Manufacturers Association (AGMA) and the International Organization for Standardization (ISO) recognise the importance of using the correct open gear lubricant so that gearing can perform as designed.
This is significant because open gear lubricant selection should be treated as an engineering decision rather than simply a purchasing decision.
The analysis can connect three areas:
Gear design → operating conditions → lubricant selection
Rather than selecting an open gear lubricant solely from a general product description, engineers can evaluate how the lubricant is expected to perform on the actual gear set.
How EHD analysis can guide open gear lubricant selection
A practical lubricant film thickness analysis can be used when evaluating an existing lubricant, comparing alternatives or investigating premature gear wear.
A typical selection process can include:
- Identify the gear application.
- Gather the gear and pinion specifications.
- Record operating speed and transmitted power.
- Determine actual tooth temperature.
- Document tooth condition and surface characteristics.
- Identify the current lubricant.
- Run the EHD calculation.
- Review the calculated film thickness and Lambda value.
- Compare the result with operating requirements and equipment condition.
- Evaluate an alternative lubricant where appropriate.
This provides a more evidence-based approach to open gear lubrication.
It can also help answer a practical maintenance question:
Is the lubricant currently being used capable of generating an appropriate protective film for this specific gear set?
Applying EHD analysis to mills and kilns
Large mills and kilns commonly use girth gears and pinions that operate under demanding loads.
These gear sets can be critical to production, so lubricant selection needs to account for actual operating conditions rather than relying exclusively on generic lubricant specifications.
For a mill or kiln, an analysis can consider parameters such as:
- Gear and pinion tooth counts
- Pinion speed
- Transmitted power
- Gear face width
- Tooth pitch
- Tooth temperature
- Pressure angle
- Tooth condition
- Current lubricant
These are among the data points Petron requests when conducting its EHD analysis.
Petron’s Gear Shield family includes open gear lubricants for mills and kilns, with products covering different application and temperature requirements. Its Products by Application portfolio identifies Gear Shield products for open gear lubrication on mills and kilns.
Applying EHD analysis to shovels and draglines
Mining shovels and draglines present another demanding open-gear application.
Their large gear sets operate under high loads and in environments where dust, temperature and other operating conditions can affect lubrication performance.
An EHD analysis can help evaluate whether a lubricant is capable of producing an appropriate film for the specific gear set.
This is particularly useful when:
- A lubricant is being changed.
- A new lubricant is being considered.
- Gear wear is occurring faster than expected.
- A reliability team wants to verify lubricant suitability.
- Operating conditions have changed.
- There is a question about whether the existing lubricant provides sufficient film thickness.
Petron lists several open-gear lubricants specifically for shovels and draglines, including Petrotac EP Premium products with different published ambient-temperature ranges.
Worked example: an illustrative EHD analysis
Consider a hypothetical open gear on an industrial mill.
Assume the engineering team records:
- A defined gear and pinion tooth count
- A specified gear face width
- A known pinion speed
- A measured tooth temperature
- A known transmitted power
- A defined tooth pitch
- A specified pressure angle
- A documented tooth condition
- An existing open gear lubricant
The data is entered into an EHD analysis.
The calculation produces an estimated lubricant film thickness and Lambda value for the specified gear set.
The reliability team can then compare that result with the expected operating requirements and examine it alongside actual tooth condition, wear patterns and lubrication history.
Now consider a second lubricant.
Instead of simply comparing the two products by viscosity or price, the same gear-set information can be used to calculate the expected EHD performance of the alternative lubricant.
This creates an application-specific comparison:
Lubricant A → calculated film thickness → Lambda
versus
Lubricant B → calculated film thickness → Lambda
The numbers in this example are intentionally illustrative. Actual film thickness and Lambda values should be calculated using the real gear geometry, lubricant properties and operating conditions rather than assumed values.
Why EHD analysis is useful for reliability teams
An open gear lubricant film thickness analysis can support several maintenance and reliability decisions.
Lubricant selection
It provides technical evidence when deciding whether an existing lubricant is appropriate or whether an alternative should be evaluated.
Wear investigation
If a gear or pinion is experiencing unexpected wear, EHD analysis can help determine whether lubricant film performance could be contributing to the problem.
Lubricant changeover
Before changing products, analysing the proposed lubricant against the actual gear set can provide additional confidence in the selection.
Preventive maintenance
Understanding the lubrication regime can complement inspections and condition-monitoring activities.
Asset-life management
Large ring and girth gears represent significant capital assets. Petron notes that such gears are typically designed for at least 25 years of service life, making the prevention of premature wear an important consideration.
When should you request an EHD film thickness analysis?
An EHD film thickness analysis is particularly useful when lubricant selection needs to be validated against a specific gear set.
Consider requesting an analysis when:
- You are evaluating a new open gear lubricant.
- You want to compare the current lubricant with an alternative.
- Gear or pinion wear is occurring prematurely.
- You are experiencing changes in operating conditions.
- You are investigating lubricant performance.
- You want technical evidence to support a lubricant change.
- You need to assess lubrication performance on a critical mill, kiln, shovel or dragline.
Petron provides an EHD analysis for specific gear sets and asks users to submit the required gear and operating information for evaluation. Petron states that its analysis can be performed quickly and at no charge.
Frequently Asked Questions
What is EHD film thickness analysis?
EHD film thickness analysis is an engineering calculation used to estimate the lubricant film generated between contacting gear surfaces under elastohydrodynamic lubrication conditions. It considers the lubricant together with gear geometry and operating conditions to determine film thickness and Lambda. Petron uses a proprietary computer program for this analysis based on the complex calculation methodology described in AGMA and ISO standards.
How is open gear lubricant film thickness calculated?
Open gear lubricant film thickness is calculated using lubricant properties together with factors such as gear geometry, speed, transmitted power, tooth temperature, pressure angle and tooth condition. The calculation produces an estimated film thickness for the specific gear-set operating conditions.
Why does lubricant film thickness matter?
Lubricant film thickness matters because sufficient film can separate contacting gear surfaces and reduce metal-to-metal contact and wear. An inadequate film can increase surface interaction and contribute to premature wear, particularly in heavily loaded gear applications.
What is an open gear lubricant film thickness calculator?
An open gear lubricant film thickness calculator is a tool intended to estimate lubricant film thickness for an open gear application. For meaningful lubricant selection, however, the calculation needs to account for the actual gear geometry, load, speed, temperature, surface condition and lubricant characteristics rather than relying on viscosity alone.
What is an EHD film thickness calculator?
lubricant film between contacting surfaces. For critical industrial gear sets, an application-specific EHD analysis is preferable to a generic calculator because the result depends on the actual gear and operating conditions.
How do you select an open gear lubricant using EHD analysis?
Select an open gear lubricant by comparing its calculated film performance against the requirements of the actual gear set. Gather the gear geometry, speed, power, tooth temperature, tooth condition and lubricant information, perform the EHD analysis, review film thickness and Lambda, and consider the result alongside OEM requirements and equipment condition.
Which equipment can benefit from EHD analysis?
Large open gears used in mills, kilns, mining shovels and draglines can benefit from EHD analysis when lubricant selection or performance needs to be evaluated. The analysis is particularly useful for critical gear sets where premature wear or an inappropriate lubricant could affect equipment reliability.
From lubricant specifications to application-specific selection
Choosing an open gear lubricant should not stop with a product data sheet.
Viscosity, adhesion, temperature range and extreme-pressure performance remain important, but they describe only part of the lubrication picture. The actual question is whether the lubricant can generate and maintain an adequate protective film under the conditions experienced by the gear.
That is where open gear lubricant film thickness analysis becomes valuable.
By combining lubricant characteristics with gear geometry, speed, load, temperature and tooth condition, EHD analysis provides a more application-specific way to evaluate lubrication performance.
For mills and kilns, mining shovels and draglines, this approach can help reliability teams make better-informed lubricant-selection decisions and investigate potential causes of premature gear wear.
Review the factors behind open gear lubricant film thickness and request an EHD analysis.