Lubrication

General guidelines
on lubrication

Lubrication is an important criterion in driven linear units. Lubricants reduce friction, minimise wear, prevent corrosion, protect against contamination and extend the operating life.

Ensuring correct function

The function of a linear unit is dependent on reliable lubrication. The units must therefore be relubricated at appropriate intervals.

The length of the interval is essentially dependent on the travel velocity, load, operating temperature, stroke length and environmental conditions. The cleaner the environment, the smaller the quantity of lubricant consumed.

The shorter the lubrication intervals, the easier it is to justify substantial expenditure on lubrication devices on economic grounds. Where the intervals are long, lubrication by hand or using semi-automatic devices can be advantageous.

Environmental protection

Any lubrication method for driven linear units involves loss of lubricant. The lubricant used must be collected and disposed of by methods that help to protect the environment.

Actuators with monorail guidance system

For the lubrication of monorail guidance systems in linear units, grease has proved effective as a lubricant. Oil lubrication is therefore not considered at this point.

Initial greasing

The guidance systems in linear units are initially greased with a high quality lithium complex soap grease KP2P–30 according to DIN 51825 and must be relubricated during operation.

The relubrication intervals are essentially dependent on:

• the carriage travel velocity
• the load
• the operating temperature
• the stroke length
• the environmental conditions and environmental influences
• the mounting position.

Grease lubrication

Greases used in linear units are subject to the following specification:

• lithium soap or lithium complex soap grease with base oil having a mineral oil base
• special anti-wear additives for loads C₀/P ＜ 8, indicated by “P” in the DIN designation KP2K–30
• base oil viscosity ISO-VG 68 to ISO-VG 150
• consistency in accordance with NLGI grade 2.

Miscibility

In general, oils with a mineral oil base and with the same classi­fication are miscible with each other. However, the viscosities should not differ by more than one ISO VG grade. The consistencies (NLGI grade) and thickener types must match. In case of doubt, please consult the grease manufacturer.

Relubrication

For relubrication, a lithium soap or lithium complex soap grease should be used that complies with the specifications stated.

In a clean environment, lubrication intervals of more than 10 000 km can be achieved. An observation period of adequate length must be allowed for precise determination of the lubrication interval.

Lubrication nipples

The position of the lubrication nipples for the individual types is given in the descriptive chapters.

Initial greasing

The carriages in actuators with monorail guidance system and toothed belt drive are sealed, have an initial greasing and can be relubricated. The ball bearings fitted in the return units of linear and clamping actuators or the tapered roller bearings in tandem actuators are sealed and lubricated for life.

Calculation of the lubrication interval

Grease operating life

Since it is not possible to calculate all the influencing factors, the precise grease operating life can only be determined under operating conditions.

However, the following approximation equation can be used to determine a guide value for many applications:

tfG	h	Guide value for grease operating life in operating hours
tf	h	Basic lubrication interval in operating hours, ➤ Figure
KP, KW, KU		Correction factors for load, stroke length and environment, see pages starting link.

Basic lubrication interval

The basic lubrication interval t_f is valid under the following conditions, ➤ Figure:

• bearing temperature ＜ +70 °C
• load ratio C₀/P = 20
• no disruptive environmental influences
• stroke ratio H_v between 10 and 50.

Speed parameter GKW

The speed parameter GKW is defined as follows:

GKW		Speed parameter
	m/min	Mean travel velocity
KLF		Bearing factor KLF = 4,5 for greased carriages of linear recirculating ball bearing and guideway assemblies KUVE and KUSE.

Calculation
of the basic lubrication interval

t_f = basic lubrication interval · GKW = speed parameter
·

 Relubrication possible ·

 Regreasing necessary

Correction factor for load K_P

The correction factor K_P takes account of the strain on the grease at a load ratio of C₀/P ＜ 20, ➤ Figure.

Correction factor for load

K_P = correction factor for load · C₀/P = load ratio

Correction factor for stroke length K_W

The correction factor K_W takes account of the distance to be lubricated, ➤ Figure. It is dependent on the stroke ratio.

Correction factor for stroke length

K_W = correction factor for stroke length · H_v = stroke ratio

Stroke ratio H_v

In the case of a stroke ratio H_v ＜ 10 or ＞ 50, the grease operating life is reduced due to the risk of fretting corrosion or loss of grease.

The stroke ratio is calculated using the following ➤ equation:

Hv		Stroke ratio
H	mm	Stroke length
Lsaddle plate	mm	Effective saddle plate length according to following table.

If the stroke length is very short, the grease operating life may be shorter than the calculated guide value. In this case, special greases are recommended – please consult Schaeffler.

Effective saddle plate length
of carriage

Correction factor
for environment K_U

The correction factor K_U takes account of shaking forces, vibrations (a cause of fretting corrosion) and shocks, see table.

Environmental influence and correction factor K_U

Relubrication interval

If the guide value for the grease operating life t_fG is less than the required operating duration of the linear unit, relubrication must be carried out.

Relubrication must be carried out at a time when the old grease can still be forced out of the carriage by the new grease.

A guide value for the relubrication interval for most applications is:

tfR	h	Guide value for relubrication interval in operating hours
tfG	h	Guide value for grease operating life in operating hours
tfE	h	Required operating duration in hours.

Relubrication quantities

The relubrication quantities are given in the descriptive chapters of the individual actuators and linear tables.

In the case of actuators with monorail guidance system and toothed belt drive, the return shaft or drive shaft bearing arrangement is sealed and lubricated for life.

Application in special conditions

In vacuum applications, lubricants with low vapourisation rates are required in order to maintain the vacuum atmosphere. In the foodstuffs sector and in clean rooms, special requirements are also placed on lubricants in relation to emissions and compatibility. For such environmental conditions, please consult Schaeffler.

Actuators with track roller guidance system

Track rollers and return units are lubricated for life, while the raceways must be lubricated.

Relubrication of the guideway raceways

The guideway raceways must be lubricated. Relubrication can be carried out by means of lubrication and wiper units. These are mounted on the carriage or integrated in the carriage. The guideway raceways are lubricated by an oil-soaked felt insert.

The felt inserts are soaked with oil that has the following characteristics:

• viscosity 460
• H1 authorisation for the food industry.

In the case of actuators with external track roller guidance system, lubrication nipples are inserted in the end faces of the carriage. They supply oil to the felt inserts.

In the case of actuators with internal track roller guidance system, the lubrication nipples or lubrication holes are located on the sides of the carriages. These allow the supply of oil to the felt inserts.

Lubrication intervals

The lubrication intervals for guideway raceways are dependent on the environmental influences. The cleaner the environment, the smaller the quantity of lubricant consumed.

The time and quantity can only be determined precisely under operating conditions since it is not possible to determine all the influences by calculation. An observation period of adequate length must be allowed.

Lubrication of track rollers

The track rollers in the carriages of actuators with track roller guidance system are greased with a high quality lithium complex soap grease according to DIN 51825-K3K–30 and are maintenance-free.

Miscibility

Oils with a mineral oil base of the same classification are miscible with each other. The viscosities must not differ by more than one ISO VG grade.

Actuators with linear ball guidance system

For the lubrication of linear ball guidance systems in linear units, grease has proved effective as a lubricant. Oil lubrication is therefore not considered at this point.

Lubrication interval of linear ball bearings

In the case of linear tables with shaft and linear ball bearings, the lubrication interval of the guidance system is dependent on the following conditions:

• temperature
• travel velocity
• stroke length
• lubricant
• environmental conditions
• mounting position.

Based on experience, the sealed linear ball bearings fitted in the shaft guidance systems of linear tables achieve their operating life with initial greasing under the following conditions:

• loads C₀/P ＞ 10
• room temperature
• travel velocity v/v_max ≦ 0,6.

If it is not possible to achieve these conditions, relubrication must be carried out.

Actuators with screw drive

Based on experience, the relubrication interval for the spindle nuts is seven hundred hours. This applies to all nuts fitted in the spindle drives of actuators and linear tables.

Only one lubrication point

If the linear units and the nut in an actuator are supplied jointly via one lubrication point, the shortest relubrication interval is taken as the defining value.

Actuators with toothed belt drive

The bearing arrangements in the return units of toothed belts are sealed and lubricated for life.