Reliability measures aren’t just for big contractors -- they can be useful to one-man operations, as well. In this blog post, we'll provide a very basic introduction to reliability measures.
Here are a few other Shop Talk Blog posts you might find of interest:
- 7 Reasons Why You Should Keep Your Machines Clean
- Hints for Inspecting a Used Mini Excavator
- #1 Cause of Final Drive Failure
To estimate the reliability of your compact equipment, it is useful to calculate (or estimate) some of the basic measures of reliability. And if you keep track of data about repairs and failures, you can get a good estimate of the reliability of each piece of equipment you depend on.
Basic Terms and Definitions
Before we dive in, there are some definitions that need to be established:
- Reliability: the probability that your equipment will be able to perform its intended tasks under specific conditions within a certain time period
- Failure: the inability of your equipment to perform its intended purpose
- Uptime: the time during which your equipment is either fully functional or ready to run
- Downtime: the opposite of uptime
From these definitions, it is apparent that a reliable piece of equipment experiences few failures and its uptime is significantly greater than its downtime.
Basic Measures of Reliability
Now that we’ve established some definitions, let's look at some of the basic measures of reliability, starting with Mean Time Between Failures.
MTBF stands for Mean Time Between Failures and represents the average time between equipment failures. Here’s how to calculate it:
MTBF = total operating time / number of failures over a period
Let's say that over the last year your skid steer loader was in operation for about 2,000 hours. Over that 2,000 hours, it failed 8 different times. The MTBF for your skid steer would be ...
MTBF = 2,000 hrs a year / 8 failures = 250 hrs or 10.42 days
So over one year it averaged about 250 hours between failures. Or, put another way, it could operate for an average of 250 hours before it failed.
MTTR is Mean Time to Repair, and it’s the average time it takes to restore your equipment to its working condition. It includes not just repair but how long it took to get your equipment to where it could be repaired, the time it took to get parts in, and how long it took to get your equipment back from a repair vendor. It’s easy to calculate:
MTTR = total repair time / number of failures over a period of time
Let's say that the 8 times your skid steer loader failed these were the repair times in days: 12, 7, 5, 4, 2.75, 0.25, 6, 5.
MTTR = 42 days / 8 failures = 3 days
So every time your skid steer loader failed it took an average of 3 days to get it running again.
Availability is the probability that your equipment will be ready to run when you need it. There are two different ways to calculate it:
Availability = Uptime / (Uptime + Downtime)
Availability = MTBF / (MTBF + MTTR)
For the skid steer loader,
Availability = 10.42 / (3 + 10.42) = 0.78
So, there is a 78% chance that your skid steer loader will be ready to run when you need it.
Why Reliability is Important
Reliability is more than just keeping repair costs down. It means having a skid steer loader that won’t break down in the middle of an important job. It means that your compact excavator will be ready to go when a client has an emergency. It means your compact track loader will be able to move loads at a reasonable speed. You can have the best equipment in the business, but if you can’t meet a critical project deadline because one of your final drive motors is out, it isn’t helping you.
Goals for Achieving Reliability
To ensure your equipment is reliable, you want to …
- Maximize MTBF
- Minimize MTTR
- Maximize Availability
One of the best ways to support the reliability of your compact equipment lies in regular maintenance. And we don't mean waiting until something goes wrong to do things like check oil levels, which is called reactive maintenance. We mean performing maintenance on a schedule before something goes wrong. This is called preventive maintenance and it will help you achieve the reliability goals for your equipment.
Uptime Elements Passport: Reliability Engineering for Maintenance, ReliabilityWeb.com