BOMI SMT flashcards
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31 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Work and power relationship
- Work equals force times distance; power equals work divided by time. One watt is one joule per second. Use these to size motors and to check whether a load exceeds nameplate rating.
- Ohm's law triangle
- Voltage equals current times resistance (V = I × R). Rearranged, current equals voltage divided by resistance and resistance equals voltage divided by current. Applies to purely resistive DC and single-phase AC loads.
- Sensible heat air formula
- Q in Btu per hour equals 1.08 times cubic feet per minute times the dry-bulb temperature difference in Fahrenheit. The constant 1.08 accounts for air density and specific heat at standard conditions.
- Total heat enthalpy formula
- Total heat in Btu per hour equals 4.5 times airflow in cubic feet per minute times the enthalpy difference in Btu per pound of dry air. Used to size coils where latent and sensible loads both matter.
- Belt drive low airflow causes
- Reduced airflow from a belt-driven fan commonly results from slipping or glazed belts, worn sheaves, an incorrect sheave ratio, or a restricted return path. Verify motor amps against nameplate before adjusting.
- Bearing temperature alarm rule
- A grease-lubricated motor bearing running more than about 40 degrees Fahrenheit (22 Celsius) above ambient warrants investigation for over-greasing, misalignment, belt tension, or early bearing failure.
- Cavitation symptoms and causes
- Cavitation causes a rattling or gravel-like noise, falling discharge pressure, and vibration. Causes include insufficient net positive suction head, a clogged suction strainer, or air entering the suction line.
- Vibration velocity acceptance bands
- For general-purpose machines, overall velocity below roughly 0.1 inches per second RMS is good, 0.1 to 0.3 is acceptable, and above 0.3 signals attention. Trending matters more than a single reading.
- Lockout tagout sequence
- Notify occupants, shut down the equipment, isolate every energy source, release stored energy such as pressure or springs, verify zero energy by test, then apply locks and tags before work begins.
- Pump affinity laws
- For a centrifugal pump, flow varies with impeller speed, head varies with the square of speed, and power varies with the cube of speed. A small speed reduction yields a large power saving.
- Fan affinity laws
- Fan airflow changes proportionally with speed, static pressure with the square of speed, and brake horsepower with the cube of speed. Used to evaluate variable-frequency drive savings on air handlers.
- Motor current unbalance rule
- Phase current unbalance above about 10 percent on a three-phase motor causes overheating and shortened life. Investigate loose connections, single-phasing, or voltage unbalance at the disconnect.
- Refrigerant superheat definition
- Superheat is the measured suction line temperature minus the saturation temperature at suction pressure. It confirms the evaporator is fully feeding refrigerant without liquid slugging the compressor.
- Refrigerant subcooling definition
- Subcooling is the saturation temperature at condensing pressure minus the liquid line temperature. Low subcooling suggests undercharge or a restriction; high subcooling suggests overcharge or a flooded condenser.
- Belt tension deflection rule
- For a typical V-belt drive, correct tension allows roughly one belt thickness of deflection at the midpoint of the longest span when moderate thumb pressure is applied. Retension new belts after break-in.
- Grease lubrication interval drivers
- Relubrication interval shortens with higher speed, higher temperature, and contamination. Doubling operating temperature above roughly 70 Celsius can cut grease life by half for each additional 15 Celsius.
- Life cycle cost definition
- Life cycle cost sums acquisition, installation, energy, maintenance, and disposal costs, all discounted to present value. The lowest total cost option is chosen, not necessarily the lowest purchase price.
- Present value formula
- Present value equals future value divided by one plus the discount rate raised to the number of periods. Used to compare costs incurred in different future years on a common basis.
- Simple payback period
- Payback in years equals the initial investment divided by annual net savings. It is a quick screening tool that ignores the time value of money and savings beyond the payback point.
- Return on investment formula
- Return on investment equals net annual savings divided by total installed cost, expressed as a percentage. Compare the result to the organization's minimum acceptable or hurdle rate.
- PM compliance metric
- Preventive maintenance compliance equals completed scheduled tasks divided by total scheduled tasks for the period, expressed as a percentage. It measures schedule execution, not equipment reliability.
- MTBF and availability
- Mean time between failures equals operating hours divided by failure count. Availability equals MTBF divided by the sum of MTBF and mean time to repair, expressed as a fraction or percentage.
- Work order required fields
- A complete work order records requester, asset identifier, priority, labor hours, parts consumed, and completion date. Missing fields break cost-per-asset reporting and backlog analysis.
- Reorder point formula
- Reorder point equals average daily usage multiplied by lead time in days, plus safety stock. Safety stock buffers demand variability and supplier delays during the replenishment window.
- Capital versus expense test
- Capitalize spending that extends useful life, increases capacity, or adds a new asset. Expense like-for-like repairs that merely restore original function, subject to the organization's capitalization threshold.
- Backlog in work weeks
- Maintenance backlog equals total open work order labor hours divided by available weekly craft hours. A backlog of roughly two to four weeks is generally considered healthy for planned work.
- Wrench time definition
- Wrench time is the percentage of paid craft hours spent on actual productive maintenance work. Typical reactive organizations run near 25 percent; world-class planned operations reach 55 to 65 percent.
- Priority matrix logic
- Work order priority combines consequence of failure with urgency of need. Safety, regulatory, and production-stopping items rank highest; cosmetic and discretionary work is deferred to backlog.
- Discount rate meaning
- The discount rate reflects the cost of capital and risk used to convert future cash flows to present value. Higher rates penalize distant savings, favoring lower first-cost options.
- Sole source versus competitive bid
- Sole-source procurement is justified by proprietary parts, sole distributor status, or emergency need, and must be documented. Routine purchases should use competitive bids to control cost.
- Contractor scope of work elements
- A maintenance contract scope states deliverables, frequency, acceptance criteria, response times, reporting requirements, insurance, and payment terms. Vague scopes produce disputes and change orders.