Last year, Grove reported revenues of $865 million and profits of $67 million.
Keystone also includes George Group Inc., a Texas-based management consulting firm. The investors own such companies as Bell and Howell and Reliant Building Products, makers of vinyl and aluminum windows.
Hollis Rafkin-Sax, spokeswoman for Keystone in New York City, said Keystone's philosophy as a long-term investor "is to buy and build."
"Any change makes people uncomfortable, but Keystone is a financial buyer, not a strategic one. It will offer Grove guidance, financial resources and an opportunity to increase efficiency. Nothing has been said about employment," Rafkin-Sax said.
William H. Foley, Grove vice president in Shady Grove, said the deal should have no major effect on the plant's operations or employment.
Grove doesn't release employment figures, but the company is believed to employ about 2,500 workers in Shady Grove and at a product support division in Chambersburg, Pa.
Worldwide, Grove has more than 4,000 employees at plants here and in England, France, Germany, Singapore and China.
"Grove is a strong, well-managed company. We had a good year last year and we expect it will be business as usual for us," Foley said.
The deal will be closed in three months, provided Keystone comes up with the money, said Patricia DeFelice, spokesman in Hanson's Woodbridge, N.J., office.
Hanson announced in June that it was seeking buyers for Grove. The asking price at the time was $655 million.
"That was just speculative," DeFelice said.
Hanson's principal businesses include aggregates and bricks. It owns the third largest producer of construction aggregates in the United States and is the United Kingdom's largest brick manufacturer.
Christopher Collins, Hanson chairman, said the sale of Grove will complete the firm's focus on building materials and eliminate its debt. He said Hanson will be able to expand and improve its business through acquisition and capital investment.
Hanson acquired Grove in 1987 as part of a $1.6 billion merger with New Jersey-based Kidde Inc. Kidde bought Grove in 1967.
Grove Worldwide is a world leader in the design and manufacture of mobile hydraulic cranes, truck-mounted cranes and self-propelled aerial work platforms. Its largest cranes weigh 200 tons and sell for more than $1 million.
Last year, Grove landed an open-ended government contract to build mobile cranes for the Army in a deal that could mean $120 million or more for the company.
The company was started in a rented garage in Shady Grove in 1947 by John L. Grove, his brother, Dwight L. Grove, and their partner, Wayne A. Nicarry. The company's first products were farm wagons.
John L. Grove left to establish JLG Industries in McConnellsburg, Pa. That company makes aerial platforms and is a direct competitor with Grove Worldwide for that product.
In December, Grove ran its 50,000th crane off the assembly line.