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PREVENTIVE AND PREDICTIVE MAINTENANCE CONCEPTS

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Preventive maintenance (PM) is a time-based or interval-based planned service to detect and prevent potential failures and extend the life of equipment. It is planned maintenance of a plant and its equipment that is designed to improve equipment life and avoid any unplanned maintenance activity. PM includes painting, lubrication, cleaning, adjusting, and minor component replacement to extend the life of equipment and facilities.

Predictive maintenance (PdM) is the application of modern analytical techniques to decrease both the cost of maintenance and production downtime by means of early identification of imminent equipment failure (achieved through monitoring changes n condition). Its purpose is to minimize breakdowns and excessive depreciation. In its simplest form, predictive maintenance can be compared to the service schedule for an automobile. It is the key to reliability and integrity for a company.

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General

Preventive maintenance has been around for many years. For instance, our ancestors sharpened their knives, packed animal fat in the hubs of wagons, cleaned pots after dying fabrics, and oiled their muskets. This is because, to keep equipment functional, maintenance is required.

In the US, the average equipment breakdown rate was about 20% in the 1950s, but in the 1990s, the rate went to about 60%. Why? The operator was taken out of the equation due to specialization. This meant fewer people were looking at the equipment less often.

Preventive maintenance is performed by:

  • The production team through daily interaction with the equipment (touching, listening, etc.). This is called Type 1 PM.
  • Maintenance through periodic and more complex tasks. This is called Type 2 PM.

Necessity for Preventive Maintenance

Why we do preventive maintenance:

  • To minimize the number of failures of critical equipment
  • To reduce the loss of production from equipment failures
  • To acquire meaningful data from the equipment history so we can make more intelligent decisions on repair, overhaul, and replacement to maximize the return on capital employed
  • To provide tasks for planning and scheduling for minimal production disruption
  • To promote better safety, health, and environmental conditions for our workforce
  • To reduce overtime costs and provide more economical use of maintenance mechanics due to working on a scheduled basis instead of an emergency basis to repair breakdowns
  • To use timely, routine repairs to bring about fewer large-scale repairs
  • To reduce product rejects, rework, and scrap through better overall equipment condition
  • To identify equipment with excessive maintenance costs, indicating the need for corrective maintenance, operator training, or replacement of obsolete equipment
  • To better care for assets and increase the life span of assets, thereby eliminating premature replacement of machinery and equipment
  • To increase the life span of equipment

Preventive maintenance includes:

  • Non-destructive testing
  • Periodic inspections
  • Lubrication tasks
  • Preplanned maintenance activities
  • Minor repairs

Major repairs are considered corrective maintenance and should be captured accordingly. Just remember that 25% of all PM is unnecessary, and 20% cause issues. Although PM can include cleaning, lubrication, testing, and scheduled replacements, the most important task in PM is inspection. Inspection helps to detect any early signs of changes in condition, which may be a warning of impending failure.

Risks of Preventive Maintenance

Preventive maintenance is not without risk. Sometimes, PM can cause failure soon after the PM is performed. Typically, the following are the types of errors or damage that can occur during PM and other types of maintenance outages:

  • Damage to nearby equipment during a PM task
  • Damage to the equipment receiving the PM task, to include such things as:
    • Damage during the performance of an inspection, repair, adjustment, or installation of a replacement part
    • Installing material that is defective, incorrectly installing a replacement part, or incorrectly reassembling material
  • Reintroducing infant mortality by installing new parts or materials (parts with early failure rates)
  • Damage due to an error in reinstalling equipment into its original location

Optimizing Preventive Maintenance

Preventive maintenance can always be improved. The following are ways to perfect maintenance:

  1. Eliminate low-value tasks (waste)
  2. Review PM-related failures shortly after they occur
  3. Replace intrusive PM tasks with non-intrusive Condition based maintenance (CBM) or predictive maintenance
  4. Clearly develop Type 1 and Type 2 PM tasks
  5. Align Environmental Health Safety (EHS) requirements with specific PM tasks
  6. Review PM procedures regularly to improve them


    Figure 1: Choosing Reactive, Preventive, or Predictive Maintenance

Making PM Successful

The key to a successful preventive maintenance program is scheduling and execution. Scheduling should be automated to the maximum extent possible, which may mean having to update your systems with meter information. Priority should be given to preventive maintenance and a very aggressive program to monitor work and ensure it is completed according to a schedule that should be in place.

Start with your most critical equipment first and use it as a showcase to prove the value of PM. Before you start, develop baseline costs and failures to be able to compare to later. Three forms of data are needed to develop PM:

  1. Manufacturer's recommendations
  2. A review of breakdowns and repairs
  3. Consultation with operators, mechanics, and supervisors

Inspections

Frequency of inspections is determined by the type of equipment, its age, its condition, and the consequences of failure. Most inspection frequencies are:

Reliability Measurement

The mean time between failures (MTBF) is a reliability measurement calculated by dividing the number of failures by the operating time. It usually is stated in hours. For most construction equipment, use meter hours. MTBF is calculated as

        • i.e.: 15 failures in 12,000 hours = MTBF is 800 hours

Maintenance field uses of MTBF are:

  1. Establishing failure-finding task frequency
  2. Aiding in the determination of whether scheduled maintenance is worth executing
  3. Assisting in determining a protective piece of equipment's desired availability

Preventive Replacements

To schedule preventive replacements, you must have these three conditions:

  1. The total cost of replacement (parts, labor, and lost production) is less than the cost of failure.
  2. You have a good idea of the average time between failures (MTBF) and can reasonably predict preventive replacement intervals.
  3. The potential consequences of failure (other than costs) are far greater than the cost and loss required by preventive replacement.
    • Predetermined parts replacement should be minimal and done only where statistical evidence clearly indicates wear-out characteristics that can be predicted.

Lubrication

Lubrication is the essential for mechanical equipment. It is science, not an art. There are three primary types of lubrication:

  • Lube oils
  • Solids (mica, graphite, molybdenum disulfide, etc.)
  • Greases (lube oil with organic or inorganic thickener)

Lubes are determined by their viscosity and their additives for service.

It only takes 20 ppm of water to halve the life of a bearing. Lube containers (drums) left outside collect moisture from the change in temperature, which can contaminate the contents. All fill containers should be sealed.

Over-greased bearings are often in more neglect than under-greased ones. Manufacturers specify lube amounts, but we often ignore them. This can lead to equipment failure. Lubrication check sheets contain the lubrication requirements of the equipment for the specified PM frequency. The mechanical, electrical, and instrumentation check sheets contain the steps required to perform a thorough examination of your equipment to identify any existing or potential equipment problems.

Improving Predictive and Preventive Maintenance

Steps to improving predictive and preventive maintenance (PPM) are as follows:

1. Acknowledge there is a issue or cost/impact of not doing PPM

  • Expensive parts shipment
  • Unscheduled downtime
  • Costly parts and materials
  • Excessive overtime
  • Missed deliveries

2. Establish a PPM or reliability policy

  • Define what PPM is and is not
  • Identify management, operation, and maintenance roles in supporting PM and PdM
  • Describe measures and monitoring processes
  • Have the policy signed and issued by management

3. Gain operational buy-in

  • Educate others on the need for commitment
  • Provide support and assistance for access to equipment
  • Provide a willingness to monitor and track metrics (results and processes)

4. Establish an equipment data record to document information

  • Model and type
  • Physical location of equipment
  • Name plate data and serial number
  • Asset or equipment number
  • Critical spare parts list
  • Update and modification records
  • Manufacture date
  • Modifications and dates

5. Assign PM type and criticality

  • Differentiate between Type 1 (operator-performed) and Type 2 (maintenance-performed ) PM tasks
  • Based on failure consequences, identify the most critical equipment
  • Determine which path is most logical: PPM, run-to-failure, or redesign

6. Develop checklists and job plans for PM

  • Train internal personnel or contract PdM services
  • Develop a consistent format
  • Identify running versus stationary for scheduling
  • Solicit feedback (actual condition), findings, comments, and corrections
  • Define pass/fail (go/no-go) criteria for objective inspections
  • Offer responses (okay, work order, adjusted)

7. Develop PM routes and work orders

  • Create a work order format; use the Computerized Maintenance Management Systems (CMMS) format
  • Organize PPM by scheduling for close proximity

8. Develop PM schedules (based on frequencies)

  • Organize based on frequency
  • *Examine manufacturer's recommendations along with failure frequencies
  • Continue to change frequencies (monitor for condition changes)

9. Maintain equipment history and develop a close-out process

  • Develop a process to record comments and findings
  • Choose findings/activities that will initiate changes/review

10. Develop a reporting process

  • Analyze PPM results
  • Display and report monthly progress
  • Track corrective actions from PPM inspections (CP, Cost of Preventive Maintenace)
  • Develop a method to respond to problems or variances
  • Have metrics in place
    1. % PM of total maintenance hours
    2. % PM schedule compliance
    3. MTBF on critical equipment
    4. % PM review (revised PM)
    5. % Corrective from PPM
    6. PM versus CM ratio

11. Organize PPM (suggested)

  • PPM discipline is improved by dedicated personnel
  • Assign most experienced tasks

A quality preventive maintenance program requires a highly motivated preventive maintenance crew. To provide proper motivation, the following activities are suggested:

  • Establish inspection and preventive maintenance as a recognized, important part of the overall maintenance program
  • Assign competent, responsible people to the preventive maintenance program
  • Monitor and follow-up on tasks to ensure quality performance and to show everyone that management does care
  • Provide training in precision maintenance on specific equipment
  • Set high standards
  • Publicize reduced costs with improved up-time and revenues, which are the result of effective preventive maintenance
  • Measure and audit

Correct information is necessary for preventive maintenance, including the activities and cost information being reported. Equipment performance, cost reports, parts applied, labor applied, and corrective work performed are all aspects of maintenance reports.

Key Stockroom Measures

Accuracy: what is listed on record versus what is actually in the stockroom

Turn rate: total issues value over the average value of the stockroom for a given period of time

Service level: number of completions versus the number of stock item attempts (the opposite of a stockout)

Turnover rate: total inventory value divided by the total annual issues

Stockout rate: number of unfilled requests divided by the total number of requests made, multiplied by 100