4.1. Labor Time Analysis
In this work, a detailed labor time study was carried out to evaluate the maintenance process of agricultural PTO shafts. No detailed studies on this subject were found in the literature prior to this work, except approaches which use lump values to calculate machine-use and maintenance-necessity [7
The differentiation according to the design of the joints (standard and wide-angle) of the PTO shafts was necessary since the complexity of the two designs results in widely differing maintenance efforts and therefore varied in labor time. A further distribution into the four work phases of assembly, search, cleaning and lubrication, as done by Achilles et al. (2018) [18
], proved to be useful and practical. Due to an implementation of a test prior to the study, the four phases could be clearly defined and proven in advance.
Depending on the users and their maintenance mentality, the cleaning phase was often skipped on the farms and thus grease zerks were lubricated without prior cleaning. It is recommended for the lubrication of bearings that the lubrication equipment be kept clean, as dust and sand can damage the bearing surfaces [26
]. There can be many reasons for not cleaning the grease zerks including a lack of knowledge about the importance of cleanliness, presumed cleanliness of the grease zerks and the aim to save time. In this context, it should be noted that the sample consists of only 35 maintenance operations. This makes it difficult to give a general statement about the temporal influence of the cleaning phase on the whole maintenance process. To give this generally valid statement for an average operation, the data set would have to be expanded to more operations over a longer period. Due to the scope of this work, a larger sample could not be realized, but this should be taken into account when a comparable study is carried out again, for example, by giving the farmer a prior instruction to specifically include the cleaning phase.
As the results show, it was useful to differentiate the lubrication process according to the type of joint. A standard joint required an average of 49 s for the whole lubrication process, while a wide-angle joint required 100 s on average.
Despite the difference in the design of the standard and wide-angle joints, some upward outliers in the time for the lubrication process of a standard joint can be seen. The usual reason for the increased lubrication time was a difficult assembly of the PTO shaft on the implement, which requires additional time of an average 70 s per lubrication. If the PTO shaft is installed with easy accessibility, the lubrication process is favored enormously. This generally shows that complete and expeditious lubrication of the PTO shaft is highly dependent on the mounting on the implement. If a PTO shaft is attached to the implement or tractor at an optimum working height and is freely accessible, a quick lubrication of the two joints on all grease zerks is possible. This is usually the case when the PTO shaft is installed above the attached drawbar, which is dependent on the manufacturer’s function and design of the tractor or implement and cannot be changed. Additionally, in some cases outliers can be seen in the search phase of the lubrication process. This leads to the presumption that the grease zerks are often hidden or difficult to access while maintaining a joint, which is the case for both standard and wide-angle joints, as Figure 4
has shown. Considering the usage of a service assistant application, the search phase could be reduced to a minimum without any outliers, as the app will visually illustrate the position of each of the grease zerks on the joints to the user. This information can be directly given, as the PTO shaft in use is exactly defined in the assistant application and the positioning is given from the manufacturer’s design of the specific PTO shaft.
Especially for wide-angle joints, the lubrication time depends on the position of the grease zerks in the joint. Modern wide-angle joints are equipped with the zerk assembled in the bearing cap as standard (Figure 7
b), which favors lubrication due to improved accessibility. If this is missing or the grease zerk is centered in the cross kit (Figure 7
a), the time for lubrication is considerably longer. Standard joints can also have grease zerks installed in the bearing caps. For optimum access to the zerk in the bearing cap, there is a special matching access hole in the shaft guard of the PTO shaft (Figure 8
). The access hole in the shaft guard of a PTO shaft in combination with a cross kit with the zerk in the bearing cap can significantly improve the ease of maintenance [25
]. By equipping PTO shafts with the zerk in the bearing cap as standard, it can be expected that maintenance procedures may be shortened in the future, making it necessary to update the labor cost calculations. In addition, this could encourage an increased willingness of farmers to properly maintain their PTO shafts. Such a structural adjustment represents an important component for increasing the lifetime of a PTO shaft through regular and proper lubrication.
Additionally, it needs to be mentioned that the maintenance procedures for wide-angle joints were delayed due to poor user knowledge of the positions and number of grease zerks. It can be assumed that the average end user does not have enough awareness of the exact number and location of grease zerks. Reasons for this may include perceived routine, no uniform markings and too few conspicuous markings, which can be supported by a service assistant application to ensure the proper lubrication of all grease zerks.
4.2. Cost Calculation
The three cost approaches used for the maintenance of PTO shafts play a major role when considering costs over a defined period. As in other industries, labor costs and the cost for consumables are rapidly increasing in agriculture [22
]. For example, lubricants have become approximately 16% more expensive over the past ten years [28
], which leads to the need to optimize the lubrication process.
A different cost, which is not considered in this study, would be an approach about the possible environmental pollution due to too much lubricant applied. The grease is squeezed into the grease zerk, through the bearings of the cross kit and partly out of the bearings again, due to too generous lubrication, and therefore released into the environment later. However, it is difficult to accurately measure determine the loss of grease into the environment. For this reason, it was necessary to refrain from quantifying any environmental damage and only contemplate direct costs of lubricating a PTO shaft in this work. To measure the environmental damage monetarily and quantitatively is very difficult, wherefore a further study is carried out as a continuation of this work.
4.3. Practical Simulations
The practical simulations show real-life applications of three different machines on actual farms. Comparative figures in the literature on the utilization of different machines could not be found.
The first practical simulation is of a large baler on a contractor-owned farm, based on a survey of contractors. The scenario of maintenance before usage is close to reality, since according to the survey, the PTO shafts on the large baler are lubricated daily before use. This maintenance strategy leads to increased consumption of grease as well as unnecessary and avoidable labor time. According to Walterscheid’s Service-Plus System and the manufacturer’s specifications, maintenance intervals of 40 h are specified, which results in a significantly more cost-effective scenario. For the large baler, “maintenance according to the manufacturer’s specifications” is therefore approximately 89% cheaper than maintenance before usage and 99% cheaper than no maintenance. As can be seen in Table 4
, labor time and grease input can also be reduced dramatically if maintenance is carried out as the manufacturer specifies. It can be assumed that the “no maintenance” scenario on PTO shafts on large balers is rather rare among contractors, as good maintenance and low downtime are prerequisites for a competitive company. The maintenance mentality here therefore tends towards the “maintenance before usage” scenario. It can be expected that the awareness to use correct amount of lubricant during PTO maintenance will increase over time, thus saving a significant amount of lubricant in the long run, which will also save resources.
For the second practical simulation, a feed mixer from an actual cattle farm was calculated, where the feed mixer is used twice a day for a total of approximately one hour, according to the survey. In this scenario, the “maintenance before usage” would be by far the most expensive maintenance scenario. Due to the daily use of the feed mixer, the daily maintenance would be one hundred times more expensive than the “maintenance according to the manufacturer’s specifications”. With maintenance intervals of 100 h for the standard PTO shaft and a usage time of one hour per day, maintenance would only be necessary every 100 days. The scenario of “maintenance before usage” is therefore not realistic in reality, while the “no maintenance” scenario with a lifetime reduction of 70% is questionable as well, due to the mild conditions while using the machine. The assumed lifetime reduction of about 70% for the “no maintenance” scenario might differ from case to case. If this reduction is considered as too high, the resulting longer lifetime may be seen as more economical but would still be far from optimum; the “no maintenance” scenario is still not advisable, as the risk of failure over longer periods of time remains increased. As the “maintenance before usage” is not a realistic scenario for the feed mixer, it can be assumed that the machine might be lubricated once a week instead of daily, which still results in higher costs than when lubricating as the manufacturer specifies. Therefore, once a week would suggest a maintenance interval of 7 h, while maintaining a feed mixer would be necessary just once every 100 days with the estimated usage time of one hour per day. This could still lead to annual costs of €101.30 (€50.64 for materials and €50.66 for labor). As weekly maintenance is the more realistic scenario for the feed mixer, this will be considered for the assumptions of possible cost reduction, reductions of labor time and grease reduction, as shown in Table 5
The practical scenario of the manure tank is, as already mentioned, calculated to be based on the average German dairy cattle farm and its production of manure [18
]. As the young stock is not considered, the absolute aggregation of liquid manure might be even higher than the approximated 1753.83 m³. Considering this would indicate a higher number of manure tanks brought to the fields than the estimated 126 trips, which would result in a longer running time for the PTO shaft. Additionally, the size of the manure tank is a major factor for the number of trips to the fields, as a manure tank with a larger volume would result in fewer trips to the fields. Here, a medium sized manure tank was in use on one of the experimental farms. Even though the data for the manure tank in this work can be considered as underestimated, Table 6
shows the cost reduction for “maintenance according to manufacturer’s specifications” at about 68% compared to the “maintenance before usage” scenario, while the cost reduction compared to the “no maintenance” scenario would be about 93%. Additionally, the reduction of labor time and grease input is remarkable.
On average, from the “no maintenance” scenario to the “maintenance according to the manufacturer’s specifications” scenario, costs can be reduced by approximately 94%, as Table 7
shows. From the “maintenance before usage” scenario to the “maintenance according to the manufacturer’s specifications” scenario, costs can be reduced by 83% on average. As there are no labor time or grease input in the “no maintenance” scenario, there are no savings when comparing to “maintenance according to the manufacturer’s specifications”. If maintenance is carried out as specified by the manufacturer instead of before usage, 82% of labor time and 86% of grease can be saved. The advantage of “maintenance according to the manufacturer’s specifications” compared to the other two maintenance scenarios is therefore unquestionable and can be visualized as seen in Figure 9
gives a good overview of the influence of different maintenance strategies on annual costs for different types of machines and for different use cases (daily vs. seasonal use) based on the test farm machines and the simulation model. As this is just a small data set of specific applications, it can be concluded that a big variety of farm machines and maintenance behaviours can be found in the area spanned between the lines of the graph in Figure 9
(grey areas). Even when maintenance is not performed daily but also not according to specifications, the effect on annual costs are drastic when compared to the optimal maintenance strategy.
Altogether it can be considered that the “maintenance according to the manufacturer’s specifications” scenario is the goal to be reached for proper maintenance without increased expenditure for labor or materials. To be able to achieve the optimal maintenance strategy, all relevant information on the product, lubrication and operation hours need to be available in a practical format. To reach this goal, tools such as the Walterscheid Connected Service Assistant can be a huge support for practical use, as the application can store information about the operation hours, the maintenance intervals for the specific joint, the number and location of grease zerks on the PTO shaft as well as the necessary amount of lubricant. Additionally, practical information about the PTO shaft such as the type and size of the PTO shaft, its serial number, the type of grease zerk (centered or in the bearing cap), the time until the next lubrication is necessary for each grease zerk and an overview of the lubrication history is given. Further information about the implement that the PTO shaft is powering, such as the manufacturer and model name, the usage as well as the environment (e.g., working on hard ground or in dust), can be stored for future reference.